1200 words and three scholarly references

1. Describe Mendel’s P, F1, and F2 generations in his experiments with pea plants.

2. Two heterozygote tongue rollers (dominant trait) have a child. What percentage of individuals would be predicted to be heterozygotes, homozygous dominant and homozygous recessive? What is the only genotype of two parents that would ensure that their offspring could not roll their tongue (recessive trait)? Explain your reasoning.

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3. A man with AB blood and a woman with O blood have a child. What is the genotype and phenotype of both the man and woman? Is it possible for them to have a child with O blood? Explain. Also, explain what type of inheritance the ABO blood system demonstrates.

4. Cystic fibrosis is an autosomal recessive disease characterized by two copies of a mutated CFTR gene. If one in 100 people in the United States have cystic fibrosis and one in 5.0505050505 people are carriers for cystic fibrosis, calculate the number of individuals that are homozygous dominant. In other words, how many people would have two copies of the normal (non-mutated) CFTR gene. Use the Hardy-Weinberg equation and explain how you determined this.

5. Red-green colorblindness is a recessive trait that is located on the X chromosome. A woman who is not colorblind but whose father is colorblind has a child with a man who is not colorblind. Predict the probability of their daughter having colorblindness, their daughter having normal vision, their son being colorblind, and their son having normal vision. Explain your reasoning.

6. One strand of a single DNA helix is labeled red while the other strand of the same DNA helix is labeled blue. This double helix DNA is replicated through the process of semi-conservative replication. Note that a completely newly synthesized strand of DNA will be white. Predict what the colors of the newly replicated DNAs (both strands) would be, explaining your prediction based on your understanding of semi-conservative replication.

7. Transcribe and translate the following sequence of DNA: TTAACGCCA. There is a mutation that resulted in AAA being inserted after G. Predict how this mutation would impact the product of translation.

8. Which of the following sequences cannot exist for a mRNA. Explain your answer.
a.   ATTGCC
b.   UTTCTTT
c.   AAAAAA
d.   CCCCC

9. Some antibiotics are used to kill bacteria by stopping the ribosome from functioning. Based on the central dogma of biology, why is this deadly for bacteria?

10. A cell has lost its ability to produce tRNA. Explain the consequences of this loss to the cell. Would this cell be able to divide? Explain.

151

Unit 2
is it all in the Genes?

Chapter 5

Molecular Genetics

Chapter 6 inheriting Genes

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153

Molecular Genetics

essentials

Melanin Pigment in skin provides a dark color

It darkens skin as sunlight penetrates its laye

DNA Replication makes more of itself passing itself onto
new generations

Mutation on a gene

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DNA provides the instructions for life, including
skin color

Tanzanian

Albinism

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154 Unit 2: Is it all in the Genes?

the Case of Out-of-place Color
“It was a terrible sight,” recalled the emergency room attending physician, Dr. Franc

Fourteen-year-old Joyce Carl had been rushed into the ER last week with bruises and
cuts on her left arm and shoulder, after a man tried to abduct her while she was walking
home from school. Joyce resisted, and the aggressor tried to cut off her arm. A group of
onlookers helped her to escape.

Even uninjured, Joyce would be a strange spectacle here in Dar es Salaam, Tanzania.
An albino, she had white skin, blonde hair, and pale blue eyes, making her stand out
dramatically among her black peers. Albinism is a noncontagious disease that is rare in
most parts of the world but fairly common in Tanzania, affecting one in 2,000 people.
Genetically inherited from both mother and father, it results from a lack of pigmenta-
tion. Eyes, skin, and hair are without color, and individuals with the disorder are highly
susceptible to skin cancers and burns.

Dr. Franc also knew that in Tanzania, as well as other parts of sub-Saharan Africa,
albinos are believed to have magical powers. It is not a compliment to their difference.
Witch doctors sell albinos’ hair, skin, bones, and internal organs on the open market as
ingredients for potions that are supposed to make people rich. With an arm going for
$2,000, about 20 Tanzanian albinos are killed each year for their body parts. This is a
great deal of money in developing economies, equivalent to over USD $200,000. Joyce
was almost a victim, and an estimated 170,000 albinos live in fear.

Dr. Franc told Joyce she would leave tomorrow morning, given that her condition
was improving. She was a friendly and well-adjusted young lady and he wished her well.
But as he was leaving the room she asked him, “Why is this happening to me?” Dr. Franc
knew that human genetics is a powerful force in society as well as in our bodies.

early ideas about Genetics
At times, a very obvious family trait is handed down from generation to generation.
Consider the distinctive facial features of the Hapsburgs, the royal family of the Austrian
Empire in Europe, which dominated the political scene there from 1282 to1918. Many
of its members had a protruding lower lip that became associated with the wealthy upper
class of old Austria (Figure 5.1).

While it was easy to observe certain physical characteristics, like the protrud-
ing lower lip, which have passed from one generation to the next, understanding how

Albinism

Is a noncontagious
disease that is
genetically inherited
and results from a lack
of pigmentation.

CheCk in

From reading this chapter, students will be able to:

• Examine how genetics affects society and our everyday lives.
• Explain the scientific development of big ideas in genetics and life’s origins.
• Describe and draw DNA structure and compare it with RNA.
• Explain the process of DNA replication.
• Use base sequences to view DNA as the universal language of genetics, and connect DNA to protein

production via the processes of transcription and translation.
• Analyze errors in gene regulation, connecting to such diseases as albinism and cancer.

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Chapter 5: Molecular Genetics 155

CheCk Up seCtiOn

There are examples of past and present discrimination in the U.S. society based on genetic differences.
Choose a particular case to research. Explain parallels you see between it and the discrimination seen
for Tanzanian albinos.

characteristics were passed on came only recently. Life arises by using information
passed down from parents. The information chemical was not discovered until much
later in our history. This chapter will explore the structure and function of this inherited
information.

Underlying the question of inherited characteristics is a more basic one: how did life
arise? Some thought we spontaneously developed without a need for parents: that life
simply arose on its own, nurtured within a womb. The theory that life could arise from
nonlife is termed spontaneous generation. This question has been pondered and answers
posed at least since the Greek philosopher Aristotle. The first recorded scientific consid-
eration for the question of how life began came from Aristotle. He believed that a male’s
semen was an imperfect mixture of materials that, when united with “female semen,”
would combine to make a more perfect human offspring. Nothing more than this was
known about how our species was formed.

Figure 5.1 Kaiserin (Empress) Maria Theresia of the Austrian Empire—The Haps-
burg Royal Family. The protruding lips of the Hapsburg family members clearly identify
them as related to one another.

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156 Unit 2: Is it all in the Genes?

Ideas on spontaneous generation went generally unchallenged for over 3,000 years.
Then, in 1677, Anton van Leeuwenhoek, the Dutch lens maker (discussed in an chapter 3),
observed living material under his newly developed microscope. He described the “little
animals or animalcules” in the many water samples he took from lakes. These are what we
now know to be microorganisms or microbes. He saw bacteria and fungi on cheese, bacte-
ria in his saliva, and sperm from his own samples (his own sperm sample he did not pub-
licly disclose due to religious rules at the time). Van Leeuwenhoek described the objects
he saw as “little eels or worms, lying all huddled up together and wriggling. …This was
for me, among all the marvels that I have discovered in nature, the most marvelous of all.”
But it was Robert Hooke who first coined the term “cell” to describe structures composing
the plants he observed, which looked to him like a monk’s cell or quarters in a monastery
(as discussed in earlier chapters).

The sperm van Leeuwenhoek and several contemporaries observed, were described
as little humans encased in a special cell with a tail. Thus, they reasoned, any resem-
blance of a child to its mother was due to her internal chemicals influencing the fetus’
development – that a fully formed human came from fathers. Figure 5.2 depicts the image
van Leeuwenhoek and his contemporaries claimed to have seen under the microscope.
We now know that these images were sperm cells with a nucleus and not a fully formed
organism. The sperm’s composition is quite a bit different from what van Leeuwenhoek
supposed. Ideas about the start of life changed over the centuries and remain a source of
public debate, as discussed in Bioethics Box 5.1.

Further experiments on plants and animals yielded a change in thinking. From work on
the pollination of flowers and trees, it became clear that both male and female parents con-
tributed to the next generation of plants. Knowledge about animal reproduction advanced
to show that it took a fusion of egg and sperm to create a new organism. This understanding
raised the question: What exactly is being inherited by the offspring? Spontaneous genera-
tion was disproved in an experiment by Louis Pasteur in the mid-1800s. In his experiment,
he constructed special flasks with elongated necks to keep out microbes. He showed that
life would appear only from other life. However, a clear mechanism explaining how organ-
isms inherited information from parents had not yet been suggeste

Gregor Mendel, an Austrian monk working in his garden in the late 1800s, studied
pea plants and their changing characteristics through successive generations. He noted
that certain patterns of inheritance emerged, and that predictions about offspring could

Animalcules

The dated term for a
microscopic animal,
we now know of as
microorganisms.

Figure 5.2 Homunculus, (little man) a future human being, preformed in a human
sperm.

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Chapter 5: Molecular Genetics 157

be made from observations about the mating parents. His studies are considered to mark
the birth of genetics and give Gregor Mendel the title of “founder of genetics” as a
modern discipline of study. Mendel’s conclusions will be discussed in detail in the next
chapter. The laws of inheritance described by Mendel can explain Joyce Carl’s albinism.

At about the same time that Mendel was making observations about patterns of
inheritance, other scientists had observed structures within cells that moved apart when
a cell divided. These structures were called chromosomes, which are compact bodies
that are inherited from one cell generation to the next (as discussed in chapter 3). Exper-
imentation showed that chromosomes were made up of two substances: proteins and
deoxyribonucleic acid (DNA).

The discovery of DNA was made by Friedrich Miescher, a German chemist who in
1869 extracted a substance from the nuclei of cells he was working with. This substance
was white, slightly acidic, and contained phosphorous; Miescher called it an organic
acid. However, little work was done on DNA for decades, because it was not known to
be the agent of heredity. Only in the mid-20th century did biochemists find out that DNA
was hereditary material; this discovery unleashed a flurry of research to determine its
molecular structure.

To do this, early in 1928, Frederick Griffith, a British microbiologist, conducted
experiments on mice to determine which material is inherited: proteins or DNA? He
studied the effects of a pathogenic (disease-causing) strain of Streptococcus pneumonia
bacteria. He unexpectedly noticed that when he injected mice with a set of different
strains of the same bacteria, sometimes they would not get sick. There were two forms
of S. pneumonia: the R- strain, which lacks a polysaccharide coat making it appear
rough and an S-strain, which had a coat, making it appear smooth in shape. He deter-
mined that those bacteria having a surrounding polysaccharide coat (the S-strain) were
pathogenic, or disease causing. The coat must have conferred some sort of protection
from the immune system of the mouse that allowed coated bacteria to cause the disease.
When Griffith heated the coated S-strains to kill them and injected the dead bacteria into
the mouse, the mouse lived. Bacteria were not able to hurt the mouse because they were
dead. When Griffiths mixed heat-killed virulent S-strain bacteria and live nonpathogenic
R-strain bacteria, however, the mouse died. How could a dead, nonpathogenic bacteria
kill the mouse? Griffith concluded that a chemical possessed by the heat-killed virulent
bacteria must be a transforming agent. It changed the living R-strain bacteria from one
type to another, into a S-strain type (see Figure 5.3). In 1944, Oswald Avery, a professor
at the Rockefeller University, isolated and analyzed this chemical, determining it to be
deoxyribonucleic acid. The story of Joyce Carl is based on this mystery material and
how it leads to skin color. We will explore the role of DNA in determining our unique
characteristics as this chapter unfolds.

Thus, it became clear, through a series of experiments in the early part of the 20th
century, that 1) DNA was inherited; and 2) DNA was the chemical that directed new
cell production as well as all cellular activities. However, it was unclear what this new
molecule looked like and how it actually worked.

Many scientists contributed their ideas and expertise to discovering the shape of
DNA. U.S. chemist Linus Pauling showed that protein chains of amino acids were helical
in shape, like a slinky, and were held together by hydrogen bonds between successive
turns. He suggested that the DNA molecule could resemble such a structure, and he
was right. X-ray diffraction showed that there were turns in the DNA molecule and that
certain chemicals, known as nitrogenous bases, occurred in regular, repeating patterns.

• Nitrogenous bases, as described in Chapter 2, are an important component of
nucleotides, the building blocks of DNA.

Chromosomes

Structures within cells
that move apart when
a cell divides.

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158 Unit 2: Is it all in the Genes?

Genetic Transformation
A

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B
C
A
A
A
D
D

Figure 5.3 Griffiths Experiment. Bacterial transformation, as show in the figure, was
the key to Griffith’s experiement. Bacteria ingest genetic material from the environ-
ment and begin to exhibit those chracteristics of that genetic material. As he studied
vaccines for pneumonia, Griffith discovered that bacteria could mutate quickly. One
dead bacterial cell (heat-killed S-cell) could be consumed by another bacteria (R-cell),
transforming it into another type of S-cell. It is a bit like a horror story, eating another
creature and becoming just like it.

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Bases adenine and thymine occurred in the same proportions, and guanine and cytosine
also occurred together in the same proportions. This finding led to further discovery
about DNA structure. The repeating patterns of the bases indicated that they must be
paired together in a certain way.

When English scientist Francis Crick and American James Watson were working in
Cambridge University in 1953, they used information from several sources to develop
a new model for DNA. The 23-year-old James Watson, a newly minted PhD traveled to
London to visit the lab of Maurice Wilkins at King’s College. There he discovered an
X-ray image of DNA taken by Rosalind Franklin, Wilkins’s colleague (Figure 5.4). Wat-
son studied the image to deduce the shape of DNA to be of a certain size and shape, and
from that developed a clue for the model. Franklin’s work indeed gave Watson the idea
for his model, but she did not receive credit in the publication describing the arrange-
ment of DNA, as described in the Bioethics Box 5.2.

Watson and Crick put all the research from the varied sources together, figuring
out just how DNA is inherited from generation to generation. This work represented the
birth of molecular genetics, a new field that united biology, chemistry, and genetics, to
study inheritance at the chemical level. Inheritance could now be explored at its most
elemental level.

Watson and Crick’s model is used to explain many aspects of chemical inheritance,
such as: the way in which DNA reproduced itself; the way it is transformed into protein
for a cell’s use; and how DNA directs the many activities within the cell. Watson and
Crick’s discovery is more than a simple description of a chemical; it is the basis for
explaining how information is passed on from generation to generation and within cells.
Their model will be used throughout this chapter to describe that information flow.

Molecular genetics

A new field that united
biology, chemistry
and genetics, to study
inheritance at the
chemical level.

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Chapter 5: Molecular Genetics 159

Figure 5.4 DNA uncoiled. DNA contains weak bonds holding the two strand of DNA together.
These break easily allowing DNA to unzip and expose the base pairs. The sugar-phosphate backbone of
the DNA molecule provides support to the structure. Note its base pairs, which, when exposed, gives
DNA its unique informational message. From Biological Perspectives, 3rd ed by BSCS.

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Dna as an inherited substance
the structure of Dna
Watson and Crick showed that DNA resembles a twisted ladder, with sugars and phosphates on the vertical
parts of the ladder and bases, making up the rungs of the ladder. The sugars and phosphates comprise the back-
bone of the DNA molecule. This type of structure is known as a “double helix.”As discussed in Chapter 2, DNA

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160 Unit 2: Is it all in the Genes?

is a nucleic acid, a macromolecule that stores information – the “code of life” – in strings
of base sequences. Each of these bases constitutes a code to guide the cell’s activities.
The exact structure of DNA, as sketched out by Watson and Crick, is shown in Figures
4.5 and 5.5. Sugars and phosphates hold the up-down portions of the molecule together
while bases pair with each other in the horizontal levels of the DNA in the figure.

The basic functional unit of a DNA molecule is the nucleotide, which is made of
three parts: a sugar backbone, either ribose, found in RNA, or deoxyribose, found in
DNA; a phosphate group, which contains four oxygen atoms bound to a central phos-
phate and is negatively charged and very acidic; and a nitrogen-containing molecule
that is the base. Bases make up the genetic code. There are four types of bases: adenine,
thymine, guanine and cytosine, commonly written as A, T, G, and C, but with U (uracil)
instead of T (thymine) in RNA. Bases are held together by weak hydrogen bonds that
are able to be taken apart relatively easily when they are used to direct the cell’s activities
or make new DNA. (Figure 5.4 and Figure 5.7 shows all of these components.) Bases
comprise a set of directions for the cell much like a set of directions used to find a par-
ticular location or address.

Just as you may arrive at the wrong place if there is a mistake in the directions, a cell
may have problems when its DNA has errors. In the case of cells, an error in a base in the
DNA sequence is called a mutation and can lead to problems for living cells. Mutations
are responsible for a number of diseases that will be treated in the next chapters. Muta-
tions are sometimes caused by environmental factors. They are an example of how the
environment has an influence on how DNA becomes expressed. For example, factors in
the environment, such as radiation or harmful substances, increase the risk of mutations
and therefore change the genetic structure (Figure 5.6). Changes in DNA’s structure also
lead to changes in organisms’ characteristics. Mutations are what led to Joyce Carl’s
albinism because a simple change in nucleotide sequence is the cause of her lighter skin.

Bases couple together specifically: adenine always pairs with thymine (A-T) and
guanine always pairs with cytosine (G-C), for example. This special coupling is called

Nucleotide

The basic functional
unit of a DNA
molecule.

Ribose

The sugar backbone
found in RNA.

Deoxyribose

The sugar backbone
found in DNA.

DNA

A long macromolecule
containing the
information code
that directs cellular
activities in living
organisms.

Adenine

A purine base that is
a component of RNA
and DNA.

Thymine

A pyrimidine base that
is found in DNA but
not RNA.

Guanine

A purine base
that functions as
a fundamental
constituent of RNA
and DNA.

Cytosine

A type of base found
in DNA.

Uracil

A pyrimidine base
that is one of
the fundamental
components of RNA.

CRiCk’s PeRsPeCTive oN DNA

Watson and Crick used their inductive abilities to gather information from
many other scientists and publicize the model we use today. Their discovery
captured the interest of the scientific community and they quickly became
celebrities. This interest helped get needed research money into their devel-
oping field of molecular biology. However, it takes a certain humble apprecia-
tion for the molecule as a thing of beauty, to really understand its meaning in
society and not the scientist’s fame. Consider the excerpt below from Francis
Crick in 1974, for his reflections on this process:

Rather than believe that Watson and Crick made the DNA structure, I
would rather stress that the structure made Watson and Crick. After all, I
was almost totally unknown at the time and Watson was regarded, in most
circles, as too bright to be really sound. But what I think is overlooked in such
arguments is the intrinsic beauty of the DNA double helix. It is the molecule
which has style, quite as much as the scientists. – Francis Crick, “The Double
Helix: A Personal View,” Nature, 26 April 1974.

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Chapter 5: Molecular Genetics 161

Figure 5.5 DNA’s structure.

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complementarity. You can remember this particular base pairing by recalling the phrase:
“AT the Grand Canyon,” where the initials AT connect adenine and thymine, and the
Grand Canyon’s initials G and C stand for linked guanine and cytosine.

Just what material do we inherit from one generation to the next? We inherit genes.
We have all heard someone saying, “It’s in your genes!” A gene is a discrete bit of data on
the DNA molecule, usually a series of nucleotides that code for information to be used
by the cell. A gene is the functional unit of heredity and the main player in information
transfer within cells. It carries the codes for all of our characteristics. Humans have
between 20,000 and 25,000 genes in their cells’ nuclei. A single gene has over 100,000
nucleotide pairs, and a DNA molecule contains over 200 million base pairs. A full set of
human DNA is estimated to contain over 3 billion base pairs, or enough information to
fit into 600,000 printed pages of 500 words each. In essence, that is equivalent to 1000
library books! How can all this information fit into a single cell when it is thousands of

Complementarity

The specific coupling
of bases.

Gene

A portion DNA
sequence serving
as the basic unit of
heredity.

Figure 5.6 Melanin mutation on the genetic code. A mutated melanin (gene #3 in
this hypothetical example) gene does not code for a proper functioning melanin pig-
ment molecule. From Biological Perspectives, 3rd ed by BSCS.

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162 Unit 2: Is it all in the Genes?

BioeThiCs Box 5.1: Why WAs DR. RosAliND FRANkliN,
A BRilliANT sCieNTisT, iGNoReD By heR ColleAGUes?

The truth behind how DNA was discovered starts with King’s College in
London, 1953. Dr. Franklin was born in 1920 and earned a doctorate in phys-
ical chemistry at the age of 26, against her father’s wishes. She worked at
Kings College, refining X-ray diffraction to produce the “Photograph 51” that
helped Watson and Crick develop their model of DNA.

When Dr. Franklin was a colleague of Maurice Wilkins, she was treated as
a mere helper and suffered gender discrimination in a male-dominated field.
Although her work was published in the same issue of the journal Nature, as
the Watson and Crick paper, in April, 1953, she was not given credit for her
contributions to the model.

Tragically, she died of ovarian cancer in 1958, at the age of 37, four years
before Watson and Crick received the Nobel Prize for their work. It is likely
that she died for the model . . . She worked hundreds of hours to perfect her
photographs of crystallized DNA, exposing herself to large doses of radiation.
The photo below shows scientist Rosalind Franklin’s X-ray image of DNA.
Most scientists are aware of her contributions today, and Dr. Franklin is given
posthumous credit in this textbook (Figure 5.7).

(a)

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Figure 5.7 a. and b. Watson and Crick Model of DNA. c. X-ray image of DNA by
Franklin. Franklin helped Watson and Crick to develop their model of DNA structure
in the 1950s.

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Chapter 5: Molecular Genetics 163

Figure 5.7 (continued)

A
A
A
T
T

G
C

P
P

Sugar
Sugar

Sugar
Sugar
P
P
Sugar
Sugar
G
G
C
C
P
P
Sugar
Sugar
A
A
T
T

Sugar-phosphate
backbone

Sugar-phosphate
backbone + base = nucleotide

Nitrogenous bases:
A: Adenine
T: Thymine
G: Guanine
C: Cytosine

Hydrogen bond3′ end

5′ end

A DNA molecule consists
of two spirally-wound sugar-
phosphate chains linked
through the hydrogen bonding
of four nitrogenous bases.
Adenine links with thymine
while guanine pairs with
cytosine.

(b)

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164 Unit 2: Is it all in the Genes?

Figure 5.7 (continued)

(c)

rs
.

times longer than the cell itself? The answer is that DNA is supercoiled, (Figure 5.8)
which means that it is packaged together very tightly around histone proteins, like a
shoelace wrapped around your fingers.

DNA is common to all living creatures: humans, bacteria, mushrooms, and oak trees
all contain the same type of macromolecules. A full set of DNA in an organism is called
its genome. In bacteria, as in all prokaryotes, the genome is naked and circular; it is not
surrounded by a nucleus and occurs as a continuous series of nucleotide bases. Eukary-
otes contain genomes packaged into discrete units as chromosomes. Each species has a
unique, set number of chromosomes. Fruit flies have only 8, corn has 20, and dogs have
78. All eukaryotes contain chromosomes in pairs and sometimes organisms of different
species have the same number of chormosomes.

Humans have a total of 46 chromosomes, with 23 pairs of them. In humans, chro-
mosomes are inherited, one from a mother and another from a father. Before a cell can
divide, chromosome pairs must double in number so that each new cell will contain a

histone

Group of basic
proteins in chromatin.

Figure 5.8 DNA supercoiling genes on DNA are coiled extensively around histone
proteins.

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Chapter 5: Molecular Genetics 165

full set. The chromosomes then move to two separate new cells during cell division.
How the DNA makes itself into two full sets was finally answered by Watson and Crick’s
model.

how Does eukaryotic Dna reproduce itself?

Mitosis

During roughly 90% of an average cell’s life cycle, it is actively conducting the many nor-
mal cell functions described in Chapter 3. This period of time is known as the interphase
for a cell. In the remaining 10% of the time, the cell divides via mitosis.

The cell cycle, or the life span phases a cell goes through, includes mitosis (see Fig-
ure 5.9). The cell cycle involves the division of the cytoplasm and nucleus to produce
two new identical daughter cells from one original cell. During interphase, a cell gets
ready for mitosis by. doubling its genetic material, increasing its number of organelles
and its cytoplasm size.

There are three phases of interphase: G1 phase, S Phase, and G2 phase (see Figure 5.9).
During the G1or growth-1 phase, a cell grows rapidly in size, forming new organelles and
proteins for future daughter cells. Centrioles, a special microtubule units used for mito-
sis, is made during G1 of interphase. During the S, or synthesis, phase, chromosomes are
synthesized to duplicate the genetic material. It is just before the start of the S phase that a
cell “decides” to divide or not. Why at this point? Because once a cell enters the S phase,
the large investment in doubling the DNA is too great to turn back; a cell must continue to
divide into two daughter cells. The decision to either become nondividing or begin DNA
synthesis depends on two major factors: 1) a cells’ cell-to-volume ratio. A cell needs a
certain amount of cytoplasm to be able to function normally. As discussed in Chapter 3,
that ratio is set to allow cells to transport materials to every region; and 2) the presence

interphase

The stage in cell
development following
two successive mitotic
or meiotic divisions

Cell cycle

The life span phases a
cell goes through.

G1 phase

A period in the cell
cycle in which a
cell grows rapidly
in size, forming
new organelles and
proteins for future
daughter cells.

s phase

A period in the cell
cycle in which DNA is
replicated.

G2 phase

A period in the cell
cycle in which growth
of the cell’s cytoplasm
and organelles is
completed and final
preparations for
mitosis takes place.

Figure 5.9 The cell cycle. Cells undergo a series of phases throughout their life
cycle; growing and dividing, with checkpoints regulating the process. From Biological
Perspectives, 3rd ed by BSCS.

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166 Unit 2: Is it all in the Genes?

of MPF, mitosis-phase promoting factor. This chemical triggers mitosis by activating pro-
teins that help in the cell-division process. Finally, during the G2 phase, growth of the cell’s
cytoplasm and organelles is completed and final preparations for mitosis takes place.

Mitosis then occurs, with cells dividing into identical new cells. Mitosis is defined
as the process by which the nucleus and nuclear components divide, resulting in two new
identical cells. Mitosis produces new cells for healing cut skin, making new organs in
an embryo, or giving rise to a newly created single-celled organism such as an Amoeba.
The stages of mitosis are shown in Figure 5.10.

Cells are constantly being reproduced in the human body. As body cells wear out,
they need to be replaced or repaired. Consider human bones: Did you know that all of
our bones are replaced every five to ten years? Bones remodel continually according to
a variety of factors: whether there is enough calcium for their development or whether
forces are placed upon the bones through exercise to build more mass, to name a couple.
Thus, cells divide to accomplish the life functions of an organism.

Figure 5.10 Stages of mitosis. A cell with four pairs of chromosomes is shown here,
dividing. The stages of mitosis and their respective characteristics are given for each phase.
From Biological Perspectives, 3rd ed by BSCS.

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Chapter 5: Molecular Genetics 167

Mitosis occurs in an orderly manner, with set steps for eukaryotic cells. The first
phase of mitosis is prophase (see Figure 5.10 to follow the steps of mitosis). Prophase is
characterized by chromatin being packaged into chromosomes in a cell’s nucleus. Chro-
matin is the thin, strewn about form of DNA in the nucleus. Chromatin condenses to
form chromosomes. Chromosomes, as dense bodies, can then be transported as discrete
packages into new cells as the cell divides. During prophase, the nucleus disintegrates
and centrioles move to opposite ends of the cell. The function of centrioles is not com-
pletely understood, but they are thought to organize a network of spindle fibers. Spindle
fibers are made up of microtubules, later used for pulling chromosomes to opposite ends
of the cell.

The next phase is metaphase, during which chromosomes line up at the middle of
the nucleus, the equator, attaching to the spindle fibers. Sister chromatids (identical
strand of the duplicated chromosomes) attach to spindle fibers via a kinetochore, which
is like a protein handle, securing chromosomes by way of a centromere, to the microtu-
bules of the spindle, as shown in Figure 5.10. During anaphase, identical chromosomes
move apart while attached to the spindle fibers. This physically divides genetic material
to opposite sides of a cell, called poles. The mechanism believed to occur is by micro-
tubules shortening, which pulls their attached chromosomes to the poles. At this point,
genetic material at the poles is identical to that of its parents.

The last phase is called the telophase, during which time there is a reversal of the
events occurring during prophase. Two new nuclei start to reform at the poles, chro-
mosomes elongate, forming chromatin once again, and the cell’s cytoplasm pinches,
forming an indentation along the equator of the cell. This final process, whereby the
division of the cytoplasm takes place, is called cytokinesis. In animal cells, the first sign
of cytokinesis is the formation of a shallow groove along the equator of the cell. This
pinching of cytoplasm is actually a cleavage furrow formed by a pulling of microfila-
ments. Quickly, much like purse strings, the cleavage furrow deepens to divide cyto-
plasm, forming two new cells. In plant, algae, fungal, and some bacterial cells, a cell
plate forms at the equator, with vesicles from the Golgi apparatus coalescing to form two
new plasma membranes and later, two new cell walls.

Prophase

A stage that is
characterized by
chromatin being
packaged into
chromosomes in a
cell’s nucleus.

Metaphase

A phase in which
chromosomes line up
at the middle of the
nucleus, the equator,
attaching to the
spindle fibers.

Anaphase

A cell division stage in
which chromosomes
split into two identical
groups and move
toward the opposite
poles of cells.

Telophase

A phase during
which time there is a
reversal of the events
occurring during
prophase.

Cytokinesis

The division of cell
cytoplasm.

A WAy To ReMeMBeR The PhAses oF MiTosis

To help you remember these phases, mitosis can be analogous to the dating
process. When your date arrives during the prophase, do you clean up your
room or keep it sloppy? Of course, you tidy up your clothes and try to make a
good impression. Much like in dating, a cell organizes its chromatin by making
it into chromosomes. Then in the middle of the date, or metaphase, you have
a good time; then you talk about too much biology, and Ana, in the anaphase,
starts running away from you. The same thing occurs in the anaphase of cell
division, in which chromosomes are running away to opposite poles of the
cell. Have you ever experienced that heinous call or even text message which
states: “It’s over between us . . . ” and dead silences or no responses show
it is really the end? This series of events is analogous to the telophase. Ana
is breaking up with you on the telephone just like the cell breaks up during
telophase. One would hope that a breakup would be via telephone and not
a mere text! This analogy may help you remember the salient aspects of the
mitotic phases.

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168 Unit 2: Is it all in the Genes?

Molecular processes during Mitosis
In order to carry out mitosis, just before the start of cell division, DNA doubles itself
during the S (synthesis) phase. This doubling process is termed replication and fol-
lows a series of specified steps. First, during the unwinding phase, the vertical strands
unwind beginning at a certain sequence of bases called the Initiation sequence. Helicase
is the enzyme that untwists the double helix so replication can occur. This untwisting is
much like a zipper unzipping. This step exposes bases within the DNA so that new bases
may be added onto the exposed strands by special enzymes called DNA polymerases.
In the next rebuilding phase, the exposed base strand allows a new layer of nucleotides
to form along the existing base sequence. Each of the single strands becomes a double
strand. To accomplish this, another set of enzymes, DNA polymerases, carry comple-
mentary bases to the exposed regions, so that, for example, whenever an A is exposed
a T binds to it and whenever a G is exposed a C binds to it. In this way, each new
strand contains an old set of nucleotides and a new set of nucleotides. The end result is
two double-stranded DNA molecules, both identical to its parent strand: half original
material and half newly placed by the rebuilding phase. Because of this half-new and
half-old DNA structure, this process of replicating DNA is termed semi-conservative.
See Figures 5.11 and 5.12, which illustrates this doubling of DNA.

Energy needed to add bases during replication is obtained by DNA polymerases
through hydrolyzing nucleoside triphosphate (a relative of ATP). DNA polymerases
move along the DNA molecule in a certain way. DNA polymerases add bases in a
certain direction on the DNA molecule, as shown in Figure 5.11. Notice that the
unzipped DNA looks like a fork, – in fact it is called a replication fork – with two
sides of the molecule exposed for adding bases. DNA polymerases can add nucle-
otides only where there are already existing nucleotides in place. A primer is laid
down to start the process. This is a segment of RNA molecules 10 nucleotides in
length. DNA polymerase in Figure 5.12 recognizes this sequence and begins add-
ing nucleotides. In this part of the replication fork, the DNA polymerase moves

initiation sequence

A sequence of bases
that starts the
unwinding of DNA
during replication.
helicase

The enzyme that
untwists the double
helix so that
replication can occur.
DNA polymerase

Special enzymes that
add new bases onto
the exposed DNA
strands.
semi- conservative
model

A mode by which
DNA replicates as
half-new and half-old
DNA.

Nucleoside
triphosphate

A molecule that
contains a nucleoside
bound to three
phosphates.

Replication fork

Molecules with both
its sides exposed for
adding bases.

(a) (b)

Figure 5.11 Semi-conservative model of DNA showing replication fork. a. During replication DNA poly-
merase lays down a new set of nucleotides in a replication fork. From Biological Perspectives, 3rd ed by BSCS. b.
The new neucleotide strand is complementary to an old strand.

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Chapter 5: Molecular Genetics 169

smoothly to produce newly added segments. As the fragments are laid down, primers
are detached and DNA ligases combine the segments of DNA to create one smooth
DNA molecule.

There are approximately six billion base pairs in our 46 chromosomes. In order
for replication to occur, multiple areas are untwisting at any one time, with nucleotides
being added constantly and rapidly. While there are so many possibilities for mutation
or error, only 1 in 100,000 errors actually occur. While this may appear like a small
number, when considering how much DNA a cell has, three billion base pairs, errors
would result in more than 120,000 mistakes every time a cell simply divides. To rem-
edy these problems, there are about 50 different types of DNA repair enzymes, which
remove mutated nucleotides and replace them with correct complementary ones. DNA
polymerases and DNA ligases both work together with DNA repair enzymes to create a
new strand of DNA, moving along the DNA molecule at a speed of about 20 base pairs
per second in humans. Of course, errors in replication still happen, leading to mutation
and gene changes. Joyce Carl experienced the effects of these mutations on select genes
controlling albinism.

DNA ligase

A type of enzyme that
joins DNA strands
together.

Figure 5.12 DNA replication showing a chemistry clip of replication fork adding complementary bases. From
Biological Perspectives, 3rd ed by BSCS.

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170 Unit 2: Is it all in the Genes?

Another example of a disorder resulting from an error in DNA replication is found
in the English peppered moth, Biston betularia (Figure 5.13).There are two varieties
of this peppered moth: light- and dark-colored. It was shown that the dark color arose
because of a rare but recurring mutation in some moths. During the period of industri-
alization in England in the 1800s, dark moths comprised almost 98% of the population
in the city of Manchester, which was known for its sooty conditions. Why might this
change in variety proportions have occurred? Studies by H.B.D Kettlewell showed that
darker moths thrived because they blended better with their polluted surroundings than
lighter colored moths. He hypothesized that lighter moths stood out to predators. This
is an example of how mutations can help some members of a species to survive. Hav-
ing a genetic variation in populations because of mutations is healthy for any species’
survival. Changing environmental conditions sometimes allow survival of at least some
individuals.

Why Go through it all? prokaryotic
Cell Division is More simple
Bacteria do not go through the many steps of mitosis to reproduce. Instead, they divide
using a process called binary fission. Genetic material is in a circular form in prokaryotes,
within what is termed a circular genome. Prokaryotes do not have a nuclear envelop to
protect their genetic material. Thus, DNA in prokaryotes is called “naked DNA,” without
a nuclear covering. Circular, naked DNA divides to form two new circular DNAs, each
attaching to two different areas of the cell membrane. As a prokaryotic cell grows, it
pulls the genetic material to opposite ends of the cell. Cytokinesis then occurs, forming
the physical separation between the cells. Separate, smaller circles of genetic material,
called plasmids, carry information for specific activities within a cell. Plasmids repli-
cate independently of the circular genome, also moving into new daughter cells.

How does genetic diversity get maintained in prokaryotes when parents are identical
genetically to offspring? Mutations during replication are a primary source of difference
between prokaryotes. Diversity is also promoted through bacterial genetic exchange or
sometimes called “bacterial sex” because DNA is getting transferred between organ-
isms. In this process, genetic material is exchanged through pili, or hair-like structures,
that connect two bacterial cells through which DNA is exchanged.

Prokaryotes have been on Earth almost since its origin – for over 3.9 billion years.
Prokaryote genetic resilience is astounding. Bacterial diversity is maintained through

Binary fission

The process by which
a cell divides directly
in hal

Circular genome

Genetic material in a
circular form found in
prokaryotes.

Figure 5.13 A light variation of Biston betularia, the English peppered moth.

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Chapter 5: Molecular Genetics 171

mutation because there are many cell divisions and many chances for errors and muta-
tions. Consider that, on average, it takes a generation, or 25 years, for humans to repro-
duce. It takes bacteria only minutes to form new organisms. This rapidity in reproduction
allows for more chances for mutation; and thus more chances for genetic variation.
In our story, Joyce suffers for her genetic variation, but without it, any species would
die off quickly, unable to withstand changing environmental conditions. Mutations can
range from being unnoticed to creating monstrous results, depending on how the gene
sequence is transmitted to the rest of the cell.

Dna is the Universal language
While Watson and Crick’s model of DNA established it as the hereditary material and
explained the mechanism for replication, it did not explain how the instructions are
carried out. The way the genetic code is read and expressed in living systems will be
discussed in this next sections. In short, we need to answer the question: “How are genes
expressed?”

Genes control the functions of any cell by giving directions or orders to carry out.
The orders are found in its nucleotide sequence that is read to allow the genes to be
expressed. How does DNA accomplish this? The four types of nucleotide bases in
DNA form a nearly infinite number of possible combinations to create a language of
information to transfer from DNA to cell structures. DNA has complex instructions for
building cells and organelles for every type of organism. All of life’s diversity depends
on this molecule and the many possible arrangements of nucleotides that create its
language.

After Watson and Crick’s model described its structure, DNA was studied to deter-
mine what it actually does. Linus Pauling reasoned that disease was due to differences in
chemicals between normal and afflicted people. In particular, Pauling studied hemoglo-
bin protein differences between people with sickle-cell anemia, carriers for the disease,
and normal individuals. Sickle-cell anemia is a disease that leads to abnormally shaped
red blood cells, poor oxygen carrying capacity, and a host of complications such as
blood clots and organ damage. Carriers for sickle-cell anemia only have one copy of the
sickle cell gene and sometimes show symptoms, but usually only under extreme circum-
stances (e.g., severe dehydration, physical exhaustion, or high altitude). Those afflicted
with the disease carry both copies of the sickle cell genes.

Pauling used electrophoresis, a process of separating organic materials based on
their electric charges, and found a difference in hemoglobin’s proteins: the hemoglobin
of normal people carries a stronger negative charge than that of sickle cell sufferers (Fig-
ure 5.14). The hemoglobin of sickle-cell carriers has a charge somewhere in between.
This was the basis for determining that genes affect protein structure. Thus, Pauling
deduced that, because proteins are found everywhere in the body doing so many varied
things, DNA must somehow affect protein structure. Pauling’s ideas eventually led to the
discovery that our genes give directions to make up to 2 million different proteins. In
the example of sickle-cell anemia, some years after Pauling, Vernon Ingram showed that
there was only one difference in proteins between sickle cell hemoglobin and normal
hemoglobin. One in 300 amino acids was changed, which was enough to reorient the
entire structure leading to a sickle-shaped red blood cell.

The carrier’s (and an afflicted person’s) hemoglobin shape was later determined
to confer some degree of immunity to a tropical disease called malaria. Malaria is an
infectious disease spread by mosquitos carrying a parasite that invades red blood cells
and reproduces in them. It causes flu-like symptoms, ranging from fever and chills to

sickle cell anemia

A disease that leads
to abnormally shaped
red blood cells, poor
oxygen carrying
capacity, and a host of
complications such as
blood clots and organ
damage.

Malaria

An infectious disease
spread by mosquitos
carrying a parasite
that invades red blood
cells and reproduces
in them.

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172 Unit 2: Is it all in the Genes?

respiratory problems, coma, and death, if left untreated. How does an immunity to malaria
manifest? Basically, the alternate hemoglobin shape of sickle cell gene holders (carriers
and afflicted individuals) causes enough 3-D changes in their red blood cells to pre-
vent the malarial parasite from getting into red blood cells and dividing. Plasmodium is
the protist that infects human red blood cells, causing this malady. Possessing gene copies
for sickle-cell anemia prevents Plasmodium from entering the human red blood cell.

There are more than 300 million cases of malaria each year, and it kills about 1
million people in Africa and Asia annually, according to the World Health Organization
2010 World Malaria Report. Sickle-cell carriers have 1/10th the likelihood of contract-
ing the most dangerous form of cerebral malaria. Thus, being a carrier for sickle-cell
anemia in tropical areas, where the disease is most likely to spread, is beneficial. The
carriers’ benefits come at a price though, because some individuals in the population are
going to have sickle-cell anemia. This is an example of how a harmful mutation, such as
sickle-cell anemia, can persist in a population: when there is a benefit to survival, such
as in this case (in warm areas affected by the disease), the mutation will continue to be
expressed for thousands of years.

What Do proteins Do?
As discussed in Chapter 2, proteins perform almost every aspect of an organism’s means
for maintaining an existence. Feel your skin – it is keratin protein that protects you.
Insects use chitin protein for their protection. An analysis of our hormones, such as
insulin, shows a variety of very specific proteins perform very specific functions. Pro-
teins are also enzymes, carriers of oxygen such as hemoglobin, and make up half of cell
membranes; they compose hair, hold cells together, receive hormones and chemicals for
cells, and move muscles, to name a few functions. Joyce Carl had albino skin coloration
because she could not produce the protein melanin, a skin-color molecule which makes
cells a darker shade. Proteins therefore express the essence of being alive because they
carry out both structural and functional aspects of life functions. Figure 5.15 shows the
varied roles that proteins play in living systems.

Melanin

The pigment that gives
color to human eyes,
hair, and skin

Figure 5.14 Genetic differences between normal and sickle-cell gene sequences lead
to changes in red blood cells shown above, with the abnormal red blood cell shaped like
a sickle. Only one base pair change (from GAG to GUG) between normal and sickle
cell DNA causes one amino acid change and, thus, the disease. From Biological Perspec-
tives, 3rd ed by BSCS.

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Chapter 5: Molecular Genetics 173

Gene expression: how proteins are Made
Gene expression is defined as the ability of a gene to carry its information to the rest
of a cell and perform its directives. The way our genes accomplish gene expression is to
make the variety of proteins described in the previous section. Gene sequences produce
proteins to carry out orders found in the genetic code.

However, not all genes are made into protein. Only about 5% of our genes give rise
to proteins. In fact, more than half of our genes contain information that simply repeats
thousands to hundreds of thousands of times, with no information to be passed on. These
sequences are noncoding, or do not produce proteins. These nucleotide sequences do not
become expressed as proteins because they are spliced out. In other words, they do not
have a chance to code for proteins and therefore influence an organism’s traits. When
genes do get expressed, however, it results in the production of protein and therefore has
the potential to affect organisms’ characteristics.

Thus, while the totality of our genes comprises our genotype, or genetic make-up,
only those genes leading to or coding for a protein will result in our observable char-
acteristics. Our protein make-up results in our observable traits, or the “way we look,”
which is termed our phenotype. Joyce Carl’s phenotype was albinism, but her genotype
led to the condition. How are there changes between the genotype and the expressed
phenotype? Simply put, in order to be expressed, a gene must be able to code for a pro-
tein. If Joyce’s gene mutation for albinism had occurred on a portion of genes that do not
code for proteins, she would not have developed albinism.

The production of proteins from DNA begins in the nucleus of eukaryotes. Eukary-
otic organisms have cells with chromosomes protected by a nuclear membrane. DNA
remains protected in the cell, which prevents potential damage to those chromosomes.
Because we know from Chapter 3 that ribosomes make proteins, where do you predict
the message will be sent from the nucleus? Yes, messages are sent from the nucleus to
the ribosomes to express a message into a protein form.

Gene expression

The ability of a gene to
carry its information
to the rest of a cell
and perform its
directives.

Genotype

The genetic makeup
of a cell.

Phenotype

The observable traits
of an organism.

Figure 5.15 Varied roles of proteins in living systems a. Roles of proteins in living sys-
tems. b. How do proteins affect our appearance? From Biological Perspectives, 3rd ed by BSCS.

(a)
(b)
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174 Unit 2: Is it all in the Genes?

Molecules that carry a message from the nucleus are called messenger RNA or
mRNA. They are single strands of nucleotides that carry coding sequences for strings
of amino acids (forming a polypeptide). As described in Chapter 2, amino acids are the
basic subunit of all proteins. This process of information transfer from a gene sequence
to a polypeptide requires two steps. The first, transcription, moves a message from the
nucleus to the cytoplasm, and the second, translation, reads that instructions forming a
chain of amino acids that reorient and constitute a protein:

1) Transcription: DNA ➔ m

RNA

2) Translation: mRNA ➔ Polypeptide à Protein

This two-step process is so important to understanding biology that it is known as the
Central Dogma of Biology (Figure 5.16). It explains how inherited material gives rise to
all our unique structures, functions, assets (like a pleasant personality or a high intelli-
gence), and liabilities (like disease).

With 20 different types of amino acids within a living cell, innumerable combina-
tions are possible for making any type of protein needed for life functions. How can so
many amino acids be made from a set of only four types of bases? The answer is that it
takes a sequence of three bases in the DNA and mRNA to “code” for a single amino acid.
When DNA codes for an amino acid, it has a specific set of instructions which match
to the production of an amino acid. Consider the DNA sequence, AAA (three adenine
bases) that codes for UUU (three uracil bases) on the mRNA molecule. UUU delivers a
phenylalanine amino acid to a growing polypeptide. The flow of information is given as
a simple equation below:

AAA ➔ UUU ➔ Phenylalanine

The set of instructions, in the case above AAA is a sequence of three bases (car-
ried by its respective nucleotides), called a triplet sequence on DNA. Its corresponding

RNA

A nucleic acid present
in living cells.

mRNA

Are molecules that
carry a message from
the nucleus.

Transcription

The first step of gene
expression in which
information in a DNA
strand is copied
into mRNA by RNA
polymerase.

Translation

The synthesis of
protein from the
information contained
in a molecule of
mRNA.

Central Dogma

A theory that explains
how inherited material
gives rise to all our
unique structures,
functions, assets, and
liabilities.

Figure 5.16 Central Dogma of Biology. Gene expression is a two-step process in
which DNA is transcribed into RNA which is then translated into proteins. From Bio-
logical Perspectives, 3rd ed by BSCS.

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Chapter 5: Molecular Genetics 175

sequence on mRNA, in our case given, UUU, is termed a codon. Figure 5.17 shows the
combinations of codons that code for their respective amino acids. A specific codon
leads to a particular amino acid placed onto a protein that forms in the process of a
gene’s expression. Thus, the order of nucleotides in the DNA and resulting mRNA
strands determine the combination of amino acids in a protein for which the genetic
material codes. There are 64 possible codons, and 61 are known to code for the 20 amino
acids in existence. The other 3 codon triplets serve as “start” and “stop” signals for the
process of protein production. Neither of these code for amino acids except for the start
codon, AUG, which also serves to code for the amino acid methionine. The sequence of
amino acids within a protein is important because it determines the three-dimensional
structure, orientation, and function of respective proteins. Use the chart in Figure 5.17
to trace the nucleotide sequences that produce their specified amino acids. Which amino
acid develops from an original triplet DNA sequence, CCT?

If you answered glycine, you correctly traced the origins of the amino acid to its
DNA code. Most of the time, amino acids are correctly coded for by their triplet and
codon sequences.

At times, however, if one specific amino acid is misplaced, an incorrectly formed
protein results. Such a scenario may lead to disease. To illustrate, consider our earlier

Codon (triplet)

Normal genetic code
in which a sequence
of three nucleotides
codes for a specific
amino acid.

Figure 5.17 Genetic code (for amino acids) table. This table shows the genetic code
in the mRNA codes for specific amino acids, during translation. Start codons and stop
codons do not specify a particular amino acid, instead they signal the cell to start or
stop translation. From Biological Perspectives, 3rd ed by BSCS.

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176 Unit 2: Is it all in the Genes?

exampler sickle-cell anemia is due to a single amino acid error. A change in a single base
(thymine to adenine) in the DNA strand leads to amino acid number 6 switching from
glutamic acid to valine. This single amino acid difference leads to the structural changes
described earlier in the hemoglobin.

The goal of transcription is to copy a sequence of nucleotide bases correctly into
an mRNA molecule. While mRNA is somewhat different than DNA (see Chapter 2 to
review the differences), it is able to match up to the DNA molecule to produce a comple-
mentary set of nucleotides. There are three phases of transcription: Initiation, Elongation,
and Termination (Figure 5.18b). The first step in initiation is to again, as in replication,
unwind the DNA molecule. Instead of DNA polymerase and helicase accomplishing this,
RNA polymerases bind to a specific sequence to unwind the DNA at certain sites and
start transcription. These special sites are called promoter sites, which are composed of
a series of base sequences. This region occurs at the beginning of each gene and ensures
that the mRNA is made from that point forward. RNA polymerase moves from the pro-
moter and along the DNA molecule, adding complementary bases in the elongation
phase. Bases are added in the same manner as during replication, with one exception:
Uracil, a complementary base is found in mRNA (which replaces thymine), is matched
with adenine bases on the DNA molecule to produce mRNA. For example, if a sequence
of DNA being copied is

ATTGCCACC

The mRNA sequence will have a complementary strand of

UAACGGUGG

Again, note that it is the same type of complementary base pairing as in replication, but
uracil is found in RNA in the place of thymine. The other base pairings remain the same.
Eventually, RNA polymerase will reach a sequence of DNA that tells it to stop, called a
termination sequence. This is the end of the gene, and RNA polymerase detaches from
the DNA strand. During the termination phase, mRNA is released from the DNA mol-
ecule and the separated DNA strands reform into a double helix. A cap and tail is then
added to the mRNA molecule before it leaves the nucleus for protection. This is called
RNA processing and protects the mRNA information, much as a cover protects a book.
The mRNA makes its journey out of the nucleus through nuclear pores because ribo-
somes are found within the cytoplasm. It was at this point in Joyce Carl’s transcription
process that her albinism first became expressed. Joyce probably had Oculocutaneous
albinism type I, which results in the transcription of a mutated gene on chromosome 15.
The code was brought out into the cytoplasm to be made into protein.

If her gene had not been transcribed, Joyce would not have developed albinism.
Some genes are not expressed into proteins causing organisms’ traits. Eukaryotic cells
process these noncoding sequences, called introns, by splicing them out and leaving
only coding sequences, exons. The message then gets sent through the pores of the
nuclear envelope into the cytoplasm to be “read” and made into proteins.

reading the Message: translation
When mRNA leaves the nucleus through nuclear pores, it attaches to ribosomes. Like
workers in a mini-factory, ribosomes work to read the message on mRNA coming out
of the nucleus. There are many “workers,” each with a specialized task in the transfer
of genetic information on mRNA into the amino-acid sequence found in all proteins.
Translation is also composed of three phases, termed Initiation, Elongation, and Ter-
mination, the same names used for the phases of transcription (Figure 5.18b). We’ve
seen that codons on mRNA either give start or stop directions or code for amino acids.

elongation

One of the three
phases of transcription
in which nucleotides
are added to the
growing RNA chain.

Termination

The phase in which
RNA polymerase will
reach a sequence of
DNA that tells it to
stop.

RNA processing

The process in which
cap and tail is added
to mRNA before it
leaves the nucleus (for
protection).

intron

A nucleotide sequence
removed by RNA
splicin

exon

A segment of RNA or
DNA that contains
information coding for
a protein.

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Chapter 5: Molecular Genetics 177

Amino acids are brought into the ribosome because they match complementary codons
on the mRNA molecule. Many amino acids have more than one codon on the mRNA
that draws it into the ribosome. For example, using the genetic code table in Figure 5.17,
the amino acid alanine is shown to have four different codons that code for it: GCA,
GCC, GCG, and GCU. The codons draw amino acids to the mRNA to form proteins.

However, mRNA does not directly produce amino acids. It requires a host of other
“workers” to make proteins. Transfer RNA (tRNA) molecules are shaped like a clo-
ver, carrying specific amino acids on one side of their shape and on the other, binding
with specific sequences on the mRNA molecule. In this way, amino acids are brought
in to match the sequence found on the mRNA molecule. The process occurs with the
assistance of the ribosomal RNA (rRNA). Ribosomes are composed of two subunits
consisting of rRNA. Ribosomal subunits have specific shapes to hold the mRNA strand
while amino acids are added together. See Figure 5.18 for the structure of tRNA, rRNA,
and mRNA.

tRNA

Small RNA molecules
that carry amino acids
to ribosomes for
protein synthesis.

rRNA

RNA component of
ribosome.

Figure 5.18 a. Methionine tRNA starts the process of translation, shown as the first to arrive at a ribo-
some in the figure. Then, other tRNAs bring amino acids to ribosomes during translation. During trans-
lation, a string of amino acids (forming a protein) is made along the surface of a ribosome. From Biological
Perspectives, 3rd ed by BSCS. Reprinted by permission. b. The steps of transcription: the movement of RNA
polymerase along a DNA molecule, making mRNA. From Biological Perspectives, 3rd ed by BSCS.

(a) ©
20

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178 Unit 2: Is it all in the Genes?

ribosome
(b)

Figure 5.18 (continued)

Initiation of translation begins when start codons (always AUG) signal the location
in the mRNA to begin translation. A tRNA carries a matching, complementary sequence
to the start codon. A start codon on the mRNA is always AUG, which matches to a par-
ticular tRNA carrying methionine. Figure 5.18a shows that the first amino acid in the
sequence being made is met (methionine) Every tRNA has a sequence of bases on it
called an anti-codon, which matches with the bases found on an mRNA. UAC is the start
anti-codon on the first tRNA to bring an amino acid, methionine, to the ribosome. UAC
matches (or is complementary with) the AUG start codon on mRNA.

As anti-codons on tRNAs match up with mRNA bases, amino acids are brought
to the ribosome. Matching tRNAs brings one amino acid after another to the mRNA
sequence. Each amino acid is linked to the next via a peptide bond. This process of elon-
gation continues, adding amino acids alongside the mRNA information strand.

• Dehydration synthesis, discussed in Chapter 2, links macromolecule subunits
together to form larger amino acid chains.

Enzymes on the ribosome catalyze the process, removing water to form peptide bonds
between amino acids. The growing polypeptide chain continues to elongate until tRNA

Anti-codon

A sequence of three
nucleotides in transfer
RNA molecule.

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Chapter 5: Molecular Genetics 179

reaches a stop codon of several types: UAA, UAG, UGA that signals the end of transla-
tion. Stop codons do not code for any amino acids. At this point along the mRNA, called
termination, tRNA, the ribosome, and mRNA detach from each other; the amino acid
polypeptide chain is released simultaneously. It is a multipart process in which many
specialized “workers” come together for a moment, guided by a message far away in the
DNA, to make a whole protein. Figure 5.19 depicts the many molecular players involved
in transcription and translation, forming proteins.

Once released from the ribosome, the polypeptide chain reconfigures and adopts its
unique three-dimensional conformations based on the chemistry of the amino acid sub-
units. Large molecules may form, such as the hemoglobin shown in Figure 5.20.

We began the chapter describing Joyce Carl’s struggle with albinism. Essentially, we
now know that albinism is a problem with the making of enzyme proteins that produce the
pigment, melanin. Melanin gives skin, eyes, and hair their color and protects the skin from
damage by ultraviolet (UV) light. It shields the nucleus of cells to prevent damage to chro-
mosomes and the vitally important DNA sequence within the nucleus. This is why sunlight
is so dangerous for people like Joyce, because the sun’s rays can mutate skin cells to make
them cancerous. Albinism is originally caused by a mutation on either chromosome num-
ber 5, 9, 11, or 15, each leading to its own type of albinism. This mutation is transcribed
and then translated into precursor enzymes that are unable to lead to normal melanin pig-
ment production. In Joyce Carl’s form of albinism, the most common in Sub-Saharan Afri-
can and African-Americans, hair is yellow or ginger in color and eyes are often gray-blue.
With limited melanin, skin often freckles with moles over time due to sun exposure.

In fact, skin color on the whole is due to only 10 different genes out of our total of
up to 25,000 genes. Skin color evolved because of environmental benefits to individuals
in the past. To illustrate, sunlight can be devastating for our skin’s health, as in the case
of the Joyce Carl or as you may have learned if you experienced a serious sunburn. UV
light has been shown to deplete folic acid from the skin. Folic acid is a very important
nutrient in a fetus’s brain and spinal cord development. Thus, in the distant past in a
world lacking nutrition, melanin provided the needed protection so that folic acid could
be spared for proper fetal development. Sunlight has been shown to deplete folic acid.
In sunnier climates, it was beneficial to have darker skin not merely to protect against
skin cancer, but to protect folic acid from sunlight and thus retain needed folic acid for
the growth and development of the next generation. Thus, evolution favored darker skin
tones in climates with more sunlight and thus, the evolution of darker skin colors.

On the other hand, sunlight facilitates the body’s production of vitamin D. Because
melanin blocks UV light, less vitamin D synthesis takes place in people with darker
skin tones. Fifty thousand years in the past, therefore, when food choices were limited,
it would have been better to have lighter skin to absorb sunlight and produce vitamin D.
However, because skin color is a result of only about 10 different genes, there are many
combinations and color types. A random shuffling of genes can result in a set of very light
color genes for one fraternal twin of a pair and a set of darker color genes for the other.
What determines “race?” and Does race even exist? are questions to ask when classifying
people based solely on the 10 genes of skin color.

Gene regulation

Not all our genes are expressed in our phenotype. Cells in the kidney do not express hair
color, for instance – there is no need. Cells produce only what is necessary, as we’ve
seen elsewhere in the text. In fact, genes are active only 5–10% of the time in a normal
living cell. The ability to shut certain genes off and turn some genes on, like a light
switch in a room, is termed gene regulation. Overproduction of materials is unnecessary.

Folic acid

A water-soluble
vitamin and a very
important nutrient in a
fetus’s brain and spinal
cord development.

vitamin D

A fat-soluble vitamin
that promotes that
is essential for the
absorption of calcium.

Gene regulation

The ability to shut
certain genes off and
turn some genes on.

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180 Unit 2: Is it all in the Genes?

Figure 5.19 Steps in the process of translation: initiation, elongation, termination (a top) and tran-
scription (at the bottom).

5′ end
A
A
A

A
A

A
U
U
U
U
G
G
G
G

C
C

C
C
C
C

G
G

G
A
C
G

Messenger RNA (mRNA)

Transfer RNA
(tRNA)

AA
AA

AA
AA
AA

Amino acid

Hydrogen bond

Growing
protein
chain

Direction of
translation

U
U
A
A
A
T
T
G
C

P P

Sugar Sugar

SugarSugar

PP
SugarSugar
G
G
C
C
P P

Sugar SugarA

A
T
U

Nuclear DNA

D
ire

ct
io

n
of

tr
an

sc
rip

tio
n

5′ end

3′ end

Nucleus

Each set of three
mRNA bases is a
codon which specifies
one amino acid

TRANSLATION

TRANSCRIPTION

NH2

CO
OH

A
U
G
C
Adenine
Guanine
Cytosine
Uracil

Symbols for
organic bases

Ribosome

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Chapter 5: Molecular Genetics 181

Figure 5.20 Hemoglobin molecule. Three-dimensional shape gives hemoglobin its
function. There are four polypeptide chains together forming a quarternary structure.
The specific orientation of amino acids allow for oxygen carrying “heme” groups to
sit within the molecule and hold enormous amounts of oxygen. The unique shape of
hemoglobin is shown in the image. In the next chapter, this shape is the impetus for the
suspense story. From Biological Perspectives, 3rd Edition by BSCS.

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182 Unit 2: Is it all in the Genes?

Remember, the cell is efficient and cheap. It spends and produces only when it is neces-
sary, a theme of this text.

There are two primary mechanisms for gene regulation. First, the promoter region,
discussed earlier in our transcription section, may be turned off. For example, the addi-
tion of chemical groups to DNA may prevent RNA polymerase from binding to DNA.
Second, DNA is wrapped around histone proteins that cover the promoter region, as
shown in Figure 5.21. When histones are chemically modified to unwind DNA strands,
the gene can be expressed. However, under tight coiling conditions, without access to the
promoter site, no transcription and thus no translation can occur.

errors in Gene regulation: a Focus on

Cancer

At times, cells divide uncontrollably when gene regulation fails. Unchecked and unreg-
ulated by their own genetic “stop” mechanisms, cells grow out of control. Abnormal,
uncontrollable cell division results in a tumor, or an abnormal growth of cells. The most
common cause of tumors is cancer. Cancer is caused by gene changes that prevent nor-
mal rates of mitosis. Cancer is a complex of over 200 related diseases and is a leading
cause of death in the world.

Cancer was first thought to be of genetic origin by Theodore Boveri, who studied
pedigrees of families with cancer and noted emerging patterns of inheritance. He thus
proposed that normal cells become cancerous when their chromosomes become altered
in some way to prevent the usual mechanisms of control over mitosis. Today, his research
is shown to be correct in its assumptions about cancer.

There are four major characteristics of cancer cells:

1) A loss of contact inhibition, which is the cell’s normal ability to come into con-
tact with its neighbors while dividing and inhibit its growth based on the limited
spacing around it;

2) Dedifferentiation, which is a loss of the specialized functions that normal cells
perform. For example, a normal kidney cell will participate in filtering materi-
als from the blood, but a cancerous kidney cell will not filter or function like a
kidney cell;

3) Loss of cellular affinity, which keeps the cell with cells that are histologically
similar to itself. A normal kidney cell, when mixed in a petri dish with liver
cells, will tend to migrate to other kidney cells. However, cancerous kidney
cells lose this affinity and attach to liver cells instead of other kidney cells. This
is a most dangerous characteristic because it allows cancer cells to metastasize,
or spread to other parts of the body. When a cell is determined to be capable of
spreading it is termed malignant. Malignant cells enter either the lymphatic sys-
tem or the blood stream to migrate to other parts of the body and grow. It is this
growth that gets in the way of other organ functions and leads to serious health
consequences and sometime death; and

Cancer

A tumor caused by an
uncontrolled division
of cells.

Contact inhibition

Cell’s normal ability
to come into contact
with its neighbors
while dividing and
inhibit its growth
based on the limited
spacing around it.

Dedifferentiation

Is the loss of the
specialized functions
that normal cells
perform.

Metastasize

The process in which
cancer cells spread
to other parts of the
body.

Malignant

The ability of a cell to
spread.

Figure 5.21 A histone protein is wrapped in DNA.

c
ie

n
c

e
p

ic
s/

Sh
u

t
te

rs
to
c
k.
c
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Chapter 5: Molecular Genetics 183

4) Immortality – Cancer cells may live for an eternity if given the right conditions
such as nutrients and water. They do not age as other cells do; instead an enzyme
called telomerase rebuilds their DNA ends, or telomeres, so that the cells may
replicate forever. In normal cells, telomeres wear out after about 100 DNA rep-
lications, at which point the cell can no longer divide, and it dies.

Is simply injecting telomerase the fountain of youth? Preliminary studies show injec-
tions of telomerase into experimental animals, instead of leading to longevity, caused
tumorogenesis, or increased tumor formation, and thus premature death. In other words,
injecting telomerase caused increased cancer rates in animals. This dampened the enthu-
siasm for telomerase as a fountain of youth. The inheritance of cancer is under some
degree of genetic control. Cancer may strike at any age and any socioeconomic class.
For example, Hollywood star Christina Applegate (see Figure 5.22) fought breast cancer
in her thirties and is now cancer-free.

summary
The chapter began with the plight of a young person, Joyce Carl, who endured social
discrimination and physical harm as a result of the natural processes described: tran-
scription, translation, mutation, and gene expression. The processes of replication and
mitosis serve to produce new, identical cells. In order for genetic material to direct the
activities of a cell, it is transcribed and translated into proteins. These proteins then act
within a cell to carry out its activities. With a change in melanin production, for exam-
ple, our chapter story shows that lives are changed within our society. The processes
described in this chapter yield our many characteristics that enable life functions. Those
traits will be described in more detail in the next chapter. It is the hope that greater
knowledge about genetics and its promise for improving human health will lead to a
more understanding world.

Telomerase

An enzyme that
rebuilds the DNA
ends of cancer cells.

Telomere

A compound structure
found at the end of a
chromosome.

BioeThiCs Box 5.2: iMMoRTAliTy oF helA Cells – siNCe
1951 AND GoiNG sTRoNG!

A best-selling book by Rebecca Skloot, The Immortal Life of Henrietta Lacks,
chronicles a unique 1951 case in which a woman’s cervical cancer cells were
harvested and grown, although without her knowledge. The patient, Henrietta
Lacks, died of cancer the same year, but given nutrients her cancer cells are
still kept alive in labs around the world, where they are used in experimenta-
tion and observation. Recently, these HeLa cells (named after the patient from
whom they were drawn) were studied to determine a relationship between
human papilloma virus and cervical cancer. This resulted in a vaccine to prevent
transmission of cervical cancer.

Of course, cancer cells do die when the host organism dies, because they
lack nutrients from the body. Cancer cells are subject to the same kinds of
needs as any other cell, but they look different. Their structure is different,
mitosis occurs more frequently, and dedifferentiation is obvious. The heirs of
Henrietta Lacks are in court to determine the legal ownership of HeLa cells
and, of course, who benefits financially from HeLa’s scientific results. The legal
results remain to be seen.

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184 Unit 2: Is it all in the Genes?

Figure 5.22 Actress Christina Applegate was diagnosed with breast cancer in 2008.

Fr
e

e
/S

h
u
tt
e
rs
to
c
k.
c
o
m

MoBy DiCk WAs AN AlBiNo

Our chapter starts with the story of human albinism in Joyce Carl. However,
albinism can occur in animals, including whales. Years before classic author
Herman Melville wrote his fictional work, Moby Dick, about a great white
whale, whalers were captivated by another great white … “Mocha dick.” This
large albino sperm whale was named after the Chilean island of Mocha in the
Pacific. From 1810 through the 1830s Mocha Dick had numerous encoun-
ters with whalers – attacking and damaging numerous ships, leaving some men
dead. Mocha Dick was likely not the only albino whale in the sea, but he was
certainly a notable inspiration to a classic tale. Our next chapter begins with a
story about vampirism, also an inherited characteristic, with a history long ago
emanating from parts of Eastern Europe.

CheCk OUt

summary key points

• Differences in traits may lead to serious social discrimination issues.
• DNA is the hereditary agent of transmission.
• Genes are sections of DNA that contain instructions for making proteins.
• Replication is the way DNA divides itself.
• Transcription is the way DNA is made into messenger RNA. Translation is the way messenger RNA

is made into proteins.
• Skin color evolved due to the benefits and drawbacks of the environments of the times.
• Cancer is an inability to regulate the genetics of mitosis.

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Chapter 5: Molecular Genetics 185

Albinism
Adenine
Anaphase
Animalcules
Anti-codon
Binary fission
Cancer
Cell cycle
Central Dogma
Chromosomes
Circular genome
Codon (triplet)
Complementarity
Contact inhibition
Cytokinesis
Cytosine
DNA
DNA Polymerase
DNA ligase
Dedifferentiation
Deoxyribose
Elongation
Exon
Folic acid
G1 phase
G2 phase
Gene
Gene expression
Gene regulation
Genotype
Guanine
Helicase
Histone

Initiation

Interphase

Intron
Malignant
Malaria
Metaphase
Metastasize
Melanin
Molecular genetics
Nucleotide
Nucleoside triphosphate
Phenotype
Prophase
Replication fork
Ribose
RNA
RNA processing
mRNA
tRNA
rRNA
S phase
Semi-conservative model
Sickle-cell anemia
Telomerase
Telomere
Telophase
Termination
Thymine
Transcription
Translation
Uracil
Vitamin D

key TeRMs

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Chapter 5: Molecular Genetics 187

Multiple Choice Questions

1. How many genes determine skin color in humans?
a. 10
b. 100
c. 1,000
d. 1,000,000

2. A person is often judged by his or her appearance. Which most affects how a person
is perceived in society?
a. exons
b.

genotype

phenotype

d. complement

3. Miescher, Griffith, and Avery each sought to explain heredity based on Mendel’s
laws. Which did they each focus upon?
a. pea plants
b. dominance
c. recessiveness
d. chemicals

4. Which of the following are the same in every DNA molecule?
a. ribose
b. ligase
c. polymerase
d. phosphate

5. Which portion of the nucleotide is most important in transmitting information?
a. deoxyribose
b. ribose
c. phosphate
d. adenine

6. Which occurs when a mismatched nucleotide is expressed in a gene sequence?
a. a changed protein
b. a changed mRNA
c. a mutation
d. all of the above

7. How many amino acids are produced from a gene sequence containing the follow-
ing bases: TTAACGCCCCTA. Assume that all of the genes are expressed as amino
acids and no noncoding or start/stop sequences are included.
a. 1
b. 4
c. 12
d. 24

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188 Unit 2: Is it all in the Genes?

8. DNA polymerase serves to:
a. lay new RNA nucleotides
b. lay new DNA nucleotides
c. fuse DNA fragments
d. fuse RNA fragments

9. Which best describes the genetic cause associated with sickle-cell anemia?
a. a missing piece of chromosome 14
b. an elongated piece of chromosome 14
c. a mutation from thymine to adenine
d. a mutation from valine to glutamic acid

10. Which most likely linked to evolutionary changes in melanin production over the
past 50,000 years of human evolution?
a. folic acid
b. Plasmodium frequency
c. complementarity of bases
d. cell affinity

short answers

1. Draw a molecule of mRNA derived from the DNA sequence: TTAGGCCACCTC.

2. List three differences between a strand of DNA and a strand of RNA.

3. Draw a diagram of the Watson and Crick double helix and label its parts, including
deoxyribose, phosphate, adenine, guanine, cytosine, and thymine. Show hydrogen
bonds using dotted lines.

4. Name two enzymes that are needed for replication. Explain the role of each enzyme
in doubling the DNA.

5. What is meant by the term “semi-conservative?” Use a drawing with colors to
explain how DNA is made using this term.

6. Explain the process of DNA transcription to a friend. What are the main results of
the process? Do the same for translation.

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Chapter 5: Molecular Genetics 189

7. Place the following terms in the correct order, starting from the beginning to the
end, tracing the flow of materials through the central dogma:
Promotor
mRNA
DNA
DNA polymerase
DNA ligase
rRNA
tRNA
Methionine
RNA polymerase

8. Transcribe the following sequence of DNA: TTAACGCC

9. Which of the following sequences cannot exist for an mRNA?
a. ATTGCC
b. UTTCCT
c. AAAATT
d. CCCCCC
Explain your answer above.

10. Antibiotics are used to kill bacteria by stopping the ribosome from functioning.
Based on the central dogma of biology, why is this so deadly for bacteria?

Biology and society Corner: Discussion Questions
1. What was Watson and Crick’s main purpose for making a model of DNA? How

does it lead to the information given in this chapter? Would we have still been able
to develop this chapter without their model of DNA? Why or why not?

2. Based on the readings in this chapter, is there such a thing as “race” in humans?
Explain your answer.

3. Who should own the rights to cells harvested from people during medical proce-
dures? Why?

4. What could be done by the Tanzanian government to prevent discrimination of Afri-
can albinos in a culture which holds beliefs that endanger their lives? How can
African albinos best improve their social integration into society?

5. Write a plan to help African albinos, who are in fear for their lives, cope with the
existing social discrimination. What are four ways albinos can improve the quality
of their lives?

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190 Unit 2: Is it all in the Genes?

Figure – Concept Map of Chapter 5 Big Ideas (Below is a sample of a concept
map for this chapter – You may draw your own in the box provided above to help
you make your personal connections)

known as

Genes organized into

divide by

confers immunity to

errors

how you
appear

genetic errors lead to
make up Mutations

leads to such as characteristics

Central Dogma of Biology

genotype
phenotype

DNA RNA Protein

transcription translation

Cancer

Replication

Albinism

Sickle Cell Anemia

Loss of
Cell
Affinity

English
Peppered
MothsEnglish

Immortality

Dedifferentiation

Loss of
Contact
Inhibition

Differences among
People in Society

Malaria

Afflicts:

235
Million

per year

causes diseases like

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191

Inheriting Genes 6

Vincent Van Gogh, the famous artist, was
believed to have been afflicted with porphyria

The porphyria gene is on a chromosome

Porphyria treatment of the future

Porphyria gene on chromosome
separates into sperm and egg

(a) Pedigree of Family with

Porphyria

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192 Unit 2: Is it all in the Genes?

the Case of the Vampire Diary
Diary entry: February 13, 2013.

Last month, during our college semester abroad, we experienced
something I will never tell another person. I can’t be sure, and maybe I am
crazy, but I know it happened… and it changed my life forever.

It all began when we spent a month in Europe. My mother’s family
came from what was once the Austro-Hungarian Empire. They immigrated
to America many years ago and did not speak much about their lives in the
old country. The town they came from, Sibiu, was now in Romania. Before
the wars, the area was German and was called Hermannstadt; before 1918,
it was in the province of Transylvania.

My friend and I rented a small car, a Yugo, and made our way to Sibiu
from Vienna. The day we left was hectic, and the sun was very bright. I did
not like the bright sun; it always made my skin ache. It was just the two

of us taking a weekend away from the rest of our class, which stayed back in Vienna.
We were friends, in a way more like acquaintances. He was bored of the party scene in
Vienna and wanted to immerse in the local culture. So he decided to accompany me to
my ancestor’s home town. He sure got what he wanted.

It was a cold night when we got to Romania, with clouds quickly moving overhead,
making the moon appear ominous. Keep in mind, I wasn’t scared at all – I had no idea of
what was yet to come. All of a sudden, the Yugo started to sputter. The car shut down and
my friend yelled, “You dummy, you forget to add gas to this thing!” I was embarrassed
and really felt bad about letting him down. I knew it was the sunlight that confused me
when I picked up the rented Yugo.

We were tired, and there were no houses along the road. “At least it isn’t snowing,”
I said meekly to try and break the cold mood between my friend and me. There was no
response as we walked through the fields. There was also no road – it had ended at an
open field with no sign of civilization. In the darkness, over on a hill in the distance, we
spotted an old house. As we came closer, it was more like a hut, with clapboard walls
and a rundown porch. I told my friend, “Let’s keep on going . . . Sibiu couldn’t be too far
off.” I knew this was a lie but I had a bad feeling about the place. There was no response,
and I knew my friend was bent on going to the house for gas.

ChECk In

From reading this chapter, students will be able to:

• Explain how inheritance of genes affects our health and society.
• Trace the discovery of laws governing heredity.
• Discuss Mendel’s experiments and the principles of genetics he derived from those experiments,

using and explaining terms such as dominant, recessive, Punnett square, and codominance.
• Describe the stages of meiosis, its products and its role in fertilization.
• Explain and give examples of single-gene characteristics in humans.
• Enumerate and explain non-Mendelian patterns of inheritance, explaining how a pedigree can be

used to trace gene flow in families.
• Use population genetics principles to trace gene flow in populations.
• List and describe the branches of gene technology, evaluating its products’ impacts on human society.

(b) A Town in Transylvania. From
Biological Perspectives, 3rd ed
by BSCS

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Chapter 6: Inheriting Genes 193

We knocked on the door, with enough force to make it heard. After a long time with
no answer, we started away. As we were leaving, an old lady opened the door. “Come in
out of the cold. You must be Americans.” I told the lady that my family had come from
this area a long time ago. The lady was the last of the Germans left in Transylvania.
“You are one of us then!” she exclaimed. She came very close to my face, looking deep
into my eyes. She remarked inappropriately, “You look like my father did when he was
young.”

As we sat in her parlor we explained that we needed just a bit of gas to get us to the
next town. The room was creepy, but the lady was very agreeable. “I’ll get my brother,
Herbie, to fetch some gas.”After she left the room, we waited and waited, but no brother.
Then, my friend felt something behind the couch – it was a man lying on the ground!
“I see you have met Herbie,” said the old lady. “He’s been drinking and needs his bed.
Would you help him up?”

This was getting to be too much, but each of us grabbed a limb to carry him. At that
moment in time, we froze, looked at each other, and knew something we dared not say –
this man was dead. His flesh was cold, and his skin bloated. Herbie’s skin was scarred,
teeth were fangs, and his face appeared almost wolf-like. He looked just like a vampire.
My friend and I looked at each other but said not a word.

We brought Herbie up to his bed and laid him down for one last rest. It was then
that he sat up, looked at us and thanked us. He looked at me and said, “You look like my
father!” I ran out of the house as fast as I could, maybe 15 miles to the town of Sibiu.

I now know that my family was from vampires; maybe their father was my grand-
father or maybe I inherited their vampire ways, somehow. But I knew one thing – I am
a vampire too.

ChECk Up sECtIon

In the story, Herbie has a blood disorder called porphyria. It is an inherited disease, occurring in about
1 in every 25,000 people. Enzymes that produce parts of his red blood cells, called hemes (which carry
oxygen) are not formed properly. More specifically, heme groups, or substances that store oxygen in
blood cells, are not formed correctly in porphyria. Without these enzymes, porphyrins (parts of hemes
in red blood cells) build up, causing lesions in the body.

Symptoms include sensitivity to light (photosensitivity); craving for blood (due to a lack of heme
groups); receding and bloody gums making teeth look like fangs; scabs and lesions from sun; organ
damage; and rampant growth of hair in body parts to appear wolf-like. Porphyria sufferers need blood
transfusions to replace their deficient hemes. We cannot be sure if the college student who narrates
the story has inherited porphyria. Its symptoms usually appear during late adolescence. However, it is
possible to manifest later in life.

Study porphyria to determine its genetic and/or environmental causes in more detail. How might
porphyria have contributed to the myth of vampires in our society? Do you think the narrator in the
story had porphyria, based on your research of the disease?

Unraveling the Mystery of Inheritance
Chapter 5 described the molecular players in gene transfer; in this chapter, we look at the
processes underlying inheritance. We begin in a small garden monastery in the 1800s.
Gregor Mendel (1822–1884), an Austrian monk who failed out of a science teaching
major in college, discovered how we pass traits onto the next generation.

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194 Unit 2: Is it all in the Genes?

By the mid-1800s, it was generally accepted that ova and sperm both contribute
genetic information to new offspring. Most biologists believed, at the time, that inheri-
tance from parents occurred as a blending of characteristics. In this view, traits from both
parents averaged together to produce new, unique offspring.

Seeking to discover if there were specific patterns in the inheritance process, Men-
del devised a set of experiments using pea plants as his subject. Using pea plants to study
inheritance was not original, but his approach to understanding how we inherit our traits
was unique. The passing of characteristics from parent to offspring is known as heredity.
Through his experiments, Mendel was able to successfully develop the basic principles
of heredity.

Mendel’s experiment was successful for a few reasons:

1) The garden pea plant Mendel chose was commercially grown at the time, repro-
duced quickly, and possessed traits easily measured by simple observation. The
garden pea plant self-pollinated, meaning that egg and sperm from the same plant
would unite. The pea plant’s sexual structures were enclosed by a petal capsule,
preventing cross-pollination from other plants. Therefore, Mendel could control
cross-breeding with select plants and not worry about accidental cross-pollination.

2) Mendel chose measurable variables to study; those that were clearly discernible.
He selected seven pea plant traits to study because they were clearly one of two
alternatives. These seven traits included shape of seeds, color of seeds, shape of
pods, color of pods, height of plant, color of flower, and position of flower. For
example, plants had either round or wrinkled peas; and either yellow or green
peas.

3) He used mathematics to measure and expose patterns in his results. Figure 6.1
shows the results of Mendel’s experiments. Note that the frequency of plant
characteristics shows distinct proportions resulting from the crosses in each
generation.

4) Mendel’s experiment was careful, logical, and sequential. His steps were metic-
ulous and well thought out. Mendel credited any successful science experiment
to certain attributes, stating in his original paper, “The value and utility of any
experiment are determined by the fitness of the material to the purpose for
which it is used.”

Heredity

The passing of
characteristics from
parent to offspring.

Figure 6.1 Gregor Mendel is the father of genetics and was an Austrian monk who
discovered the laws governing patterns of inheritance.

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Chapter 6: Inheriting Genes 195

While Mendel’s findings were groundbreaking, they were unrecognized for 35 years.
In 1865, Mendel reported his experiments and results in a paper presented to the Nat-
ural Historical Society in Bruenn (now Brno, Czech Republic), the Austro-Hungarian
Empire. Scientists in the audience dismissed his findings. Afterward, Mendel returned
to his monastery, tending to his priestly duties; he was ignored and unappreciated by the
scientific community. It was not until after his death, in 1900, that biologists began to
build upon Mendel’s paper. Mendel’s work eventually was recognized and discredited
the blending of traits perspective. Instead, his findings showed that traits were inherited
as discrete units from each parent. Mendel thus began a scientific revolution in the field
of genetics. Gregor Mendel is now recognized as the father of genetics for his work on
pea plants. Let’s take a closer look at the laws of heredity that Mendel formulated so
long ago.

Mendel’s laws
law of Dominance
Let’s revisit Mendel’s work: First, he chose to crossbreed certain plants. For example,
Mendel noted that one variety of plant always produced yellow peas, while another pro-
duced green. He took the male anther portion of a yellow pea plant and dusted the
female stigma of a green. He called these original parents the F0 generation. When he
crossed the two, all of the offspring were still yellow and none of them were green. This
first cross Mendel called the F1 generation. Any trait that appeared in the F1 generation
he called dominant for that characteristic. He surmised that any dominant trait covers up
the alternative characteristic of an organism in the F1 generation. Next, Mendel crossed
the organisms in the F1 generation in the same manner and their offspring were analyzed,
the F2 generation. Mendel conducted what is now termed a monohybrid cross. This is a
mating between two organisms, each having both characteristics for a particular trait – in
this case both yellow and green. It is termed “mono-” because it looks at the inheritance
of only one trait. Mendel surmised that although all of the plants of the F1 generation
were yellow, they harbored a hidden green characteristic able to be given to offspring.

Mendel formed a hypothesis: the covered-up trait would reappear in the F2 gen-
eration. Indeed, he predicted correctly that some offspring of all-yellow plants would
be green. He was correct; the covered-up trait always reappeared in the F2 generation,
bred from parents that did not exhibit the trait. The idea that a dominant trait covers up
another is known as the law of dominance. He called the characteristic that is covered up
the recessive trait. In his experiment, the yellow trait was dominant, and the green trait
was recessive. The original parents, the F0 generation, he deduced, were each pure – the
yellow parent had only dominant yellow characteristics and the green parent had only
green characteristics – but that these characteristics would pass along in a predictable
manner through each generation.

law of segregation
When Mendel analyzed the F2 generation, he found that a certain proportion always
appeared in his data. Note in Figure 6.2 that the F2 generation for all seven characteris-
tics he chose had a roughly 3:1 ratio of dominant to recessive characteristics.

Because the appearance and disappearance of traits occurred in constant propor-
tions, Mendel inferred that traits must be inherited as two separate, discrete units. We
now call these units alleles. Alleles are alternate forms of the same trait. For example,
if a pea has a yellow or green color possible, then either a yellow or green allele is

Dominant

The trait that covers
up other forms of the
characteristi

Monohybrid cross

The mating between
two organisms,
each having both
characteristics for a
particular trait.

Law of dominance

The idea that a
dominant trait
covers up another.

Recessive

The trait that is
covered up by a
dominant trait.

Alleles

An alternative form
of the same trait.

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196 Unit 2: Is it all in the Genes?

responsible for it. The hypothesis that there are two separate, discrete alleles that could
be inherited separately is known as Mendel’s law of segregation.

We are now able to trace the movement of alleles from parent to offspring using
a Punnett square. While this square was not actually used by Mendel, it derives from
Mendel’s law of segregation. A Punnett square is a diagram based on the law of segre-
gation that is used to predict the probability of inheritance of alleles between parent and
offspring. Figure 6.3 uses a Punnett square to show how alleles are discretely passed on
to a new generation. The mother’s alleles appear on one side of the box and the father’s
on the other side. Each parent in the figure has two possible alleles based on their
genetic make-up. Capitalized alleles are dominant and lower case alleles are recessive
in the Punnett Square. Alleles from each parent have a 50:50 chance of segregating
into an egg or sperm, eventually forming a new organism with a new genetic make-up.

• To recall from Chapter 5, an organism’s genetic make-up is known as its geno-
type. The expression of that genotype is an organism’s phenotype. In other words,
how an organism appears is its phenotype; and what comprises inside an organ-
ism’s genes is its genotype.

The Punnett square gives the probability of producing an organism with a particular
genotype within each box. Each box of the Punnett square represents a 25% chance that
an organism’s genotype will appear in the offspring generation. Figure 6.4 shows the
process of allele transfer between parents to offspring in porphyria. In our story, acute
intermittent porphyria (AIP) is a dominant trait, meaning that if a person has one allele
for it then he or she will have the disease.

law of Independent assortment
In another set of experiments, called a dihybrid cross, Mendel mated plants tracing
two different traits – pea shape and color. In a dihybrid cross both parents possess
dominant and recessive characteristics for a particular trait. It traces the inheritance
of two separate traits at the same time. The term “di-” is used because it looks at the

Law of segregation

The hypothesis that
states that there are
two separate, discrete
alleles that could be
inherited separately.

Porphyria

An inherited disease
which is characterized
by abnormal
metabolism of the
blood hemoglobin.

P generation

Long × Short

Purple × White Flowers

Axial × Terminal flowers

Green × yellow pods

Smooth × constricted pods

Yellow × Green seeds

Round × Wrinkled seeds

Total

All Long 787 Long, 277 Short 2.84:1

2.82:1

2.95:1

2.96:1

2.98:1

3.01:1

3.15:1

3.14:1

705 Purple, 224 White

651 Axial, 207 Terminal

428 Green,152 Yellow

882 Smooth, 299 Constricted

6,022 Yellow, 2,001 Green

5,474 Round, 1,850 Wrinkled

14,949 Dominant,
5,010 Recessive

All Purple

All Axial

All Green

All Smooth

All Yellow

All Round

All Dominant

F1 generation F2 generation Ratio

Figure 6.2 Results of Mendel’s experiments with pea plants F1 and F2 generations.

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Chapter 6: Inheriting Genes 197

Figure 6.3 Principle of segregation. Alleles separate into opposite ends of the cell.
Alleles move independently of each other, according to Mendel’s laws, with equal
chances of being transmitted to offspring. Note that any one of the four gametes
produced by parents in the figure could be transmitted to the offspring generation.
The Punnett square shows the relative probability for each gamete to give rise to its
genotype.

F1 generation

F2 generation

9/16 Yellow and round
(1 YYRR, 2 YyRR, 4 YyRr, 2 YYRr)

3/16 Green and round
(1 yyRR, 2 yyRr)

3/16 Yellow and wrinkled
(1 YYrr, 2 Yyrr)

1/16 Green and wrinkled
(1 yyrr)

All yellow and round
(YyRr)

Yellow and round

YYRR

Green and
wrinkled

yyrr

YYRR x yyrr YyRr x

YyRr

YYRR YYRr YyRR YyRr

YR Yr yR

YYRr YYrr YyRr

Yyrr

YyRR YyRr yyRR

yyRr

YyRr

YyRr
YR

yr Yyrr yyRr yyrr

Figure 6.4 a. A Punnett square for porphyria, a cross is shown between a parent
with a dominant gene for porphyria and a normal parent for the F1 generation. The
Punnett square shows that 50% of offspring will exhibit the disease. b. The disease
porphyria is the likely basis for the legend of vampires.

(a) (b)

P p

p
p
P
P

p p

p pP P

Parent with
Porphyria

50% of offspring have the
disease

normal
parent

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198 Unit 2: Is it all in the Genes?

inheritance of two traits. One organism had yellow, smooth peas while the other had
green, wrinkled peas. As shown in Figure 6.5, yellow and smooth are dominant traits,
while green and round are recessive. When he analyzed the offspring of these crosses
(the F1 generation), Mendel determined that traits were not inherited together. Instead
they independently assorted as they were passed from one generation to the next. Wrin-
kled and round were found alongside smooth and green. All of the possible types of pea
plants showed up in the F1 generation of this dihybrid cross. In fact, these organisms
also showed a pattern of proportions in a 9:3:3:1 ratio, with the dominant traits occur-
ring most frequently (Figure 6.5). All new combinations of traits appeared in the next
generation, each inherited separately from each other. The idea that each pair of alleles
is sorted independently when sperm and egg are formed is known as Mendel’s law of
independent assortment.

Two factors are inherited separately, one from a mother and one from a father. Thus,
once together, they occur as either an identical pair or as a pair with different compo-
nents. When a pair of alleles is the same, they are called homozygous. When both are
dominant forms, they are homozygous dominant. When both are recessive, they are
homozygous recessive. When alleles in a pair are different from each other, they are
called heterozygous or hybrids for that trait.

In the porphyria case seen in our story, the disease is held on a dominant allele.
Thus, if a person possesses an allele for porphyria, whether homozygous dominant or
heterozygous, he or she will get the disease (see Figure 6.4). Only a homozygous reces-
sive individual does not exhibit the disease. If P = the allele for porphyria and p = the
allele for the normal condition, then an individual with PP, homozygous dominant or Pp,
heterozygous will have the porphyria trait. Only a homozygous recessive, pp will have a
normal blood condition.

Law of independent
assortment

The idea which
tells that each pair
of alleles is sorted
independently when
sperm and egg are
formed.

Homozygous

The condition in which
a pair of alleles is the
same.

Heterozygous

The condition in
which alleles a pair are
different from each
other.

seed
shape

seed
color

seed-
coat
color

pod
shape

pod
color

flower
position

stem
length

round yellow colored inflated green on sides long

wrinkled green white constricted yellow at end short

(a) ©
Ke

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ll
H
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m
p
a
n
y

Figure 6.5 a. traits of pea plants, with the top row dominant and the bottom row
recessive; b. Punnett square for a dihybrid cross for pea color and shape in pea plants.
A cross is shown between two parents with both traits. The Punnett square below
shows a 9:3:3:1 ratio in offspring characteristics.

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Chapter 6: Inheriting Genes 199

First-generation
plants

When gametes are produced
via meiosis all are YR.

When gametes are produced
via meiosis all are yr.

a. b.

YR YR YR YR yr yr yr yr

When fertilization occurs, all possible combinations
of gametes result in one kind of zygote.

YyRr YyRr YyRr YyRr

When these zygotes grow via meiosis into plants
all body cells have YyRr genotype, therefore all produce
yellow-round seeds.

These plants produce four kinds of gametes via meiosise.

YR Yr yR yr

yYrR

yYRR

YYrR

YYRR

YYRr

YyRR

YyRr

YyrR

YYrr

yYRr

yYrr

yyrR

yyRR

yyRr
Yyrr
yyrr
YR
Yr
yR

yr YR

Yr
yR
yr

When random
fertilization occurs,
16 kinds of zygotes
are produced

When these zygotes grow into plants,
they produce third-generation plants with
a 9:3:3:1 ratio of seed colors and shapes.

} }

}

Yellow round Yellow
wrinkled

Green
round

Green
wrinkled

= sperm

= egg

Second-generation
plants

Third-generation
plants

9 3 3 1

YR Yr yR yr
}
(b)

Figure 6.5 (continued)

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200 Unit 2: Is it all in the Genes?

testcross
How do we determine the genotype of an organism?—, it is not always obvious from
its appearance. Consider a green pea plant that inherits two green recessive alleles, one
from each parent. It is green in its phenotype, indicating that it inherited two green
alleles. If the plant had inherited one green and one yellow allele, it would have been
yellow. When a yellow plant appears, it is more difficult to know its genotype without
knowing its history. A yellow pea plant has a dominant allele, but does it have a recessive
that is covered up, or is the other allele dominant as well?

Through using a testcross, the genotype of the yellow pea plant is explored. In a
testcross, a known homozygous recessive organism, for example, a green pea plant is
crossed with a yellow phenotype. The green pea plant, we know, has two green (reces-
sive) alleles. But what is the yellow plant’s genotype? In this case, we do not whether
the plant with yellow peas is homozygous dominant or heterozygous. In this testcross,
a hidden recessive is most likely to be revealed. The homozygous recessive individual
(the green plant) has the best chance of passing all its recessives to the next genera-
tion. Figure 6.6 shows a testcross between a yellow pea plant and a green pea plant to
determine whether or not green peas will result in their offspring. The testcross helps to
determine the true genotype of the yellow pea plant.

The appearance of a recessive in the testcross’s progeny is the only definite proof
that the unknown genotype was indeed a heterozygote. In other words, if one of its off-
spring is green, then alleles coming from both parents must have been green. For the
new offspring to have become a homozygous recessive, one recessive allele had to come
from the yellow parent plant. However, if there is no individual with a recessive trait
for pea color in the F1 generation, it may mean simply that the recessive allele may get
expressed in another generation. Perhaps that recessive allele simply did not get passed
along this time around. It is impossible to know for sure, but a testcross gives the best
chance of being able to reveal the true genotype of an individual with a dominant phe-
notype, such as the yellow plant.

This is also the reason recessives are so difficult to study and/or remove from a
group. They are hidden, and only chance dictates whether or not an allele will become
expressed. Many times recessive traits are deleterious, or cause harm to an organism hav-
ing them. Many diseases are recessives and it may take several generations for a reces-
sive disease to appear. It is hard to track recessives for this reason. For example, a family
may be surprised that sickle-cell anemia is in their genetic history. Family members

Testcross

A known homozygous
recessive organism
is mated with a
dominant organism.

Figure 6.6 Testcross for color on pea plants. A testcross always uses a homozygous
recessive to attempt to reveal the recessive of its dominant mating partner. If even one
of the offspring shows recessive characteristics, then the dominant partner harbors a
recessive allele. In the example shown Y = yellow and y = green coloration in pea plants.

Y ?

y
y

Y
y y ?

y ?Y y

unknown
genotype

yy = if a green
phenotype shows up,
the unknown genotype
contained a recessive
allele ©

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Chapter 6: Inheriting Genes 201

may think it is not a risk because no one has had sickle-cell anemia, for as long as they
can remember. However, it may have been hidden in the heterozygote condition for a
period of time and was unexpressed. Deleterious recessives are a difficult but common
thread in most groups. Some forms of porphyria are recessive, showing up many gen-
erations after they are thought to be gone. Did our narrator in the story experience the
reappearance of a long-silent recessive allele?

Meiosis: how sex Cells are Formed
We have seen how traits are passed on from one generation to the next; now we will
examine how organisms reproduce sexually. During Mendel’s time, it was accepted that
parents transfer their hereditary information through a process called reproduction, to
form a new organism. The central step in reproduction is fertilization, when a male and
a female sex cell, both called gametes, unite. The female sex cell is the egg; the male,
the sperm. Each contributes half the total genetic material that unites and recombines
in the zygote. If the offspring receives genetic material from both parents, how is it that
the offspring contains the same number of chromosomes as the parents? The answer is
meiosis, which is a special form of cell division in which the newly produced daughter
cells contain only half the number of chromosomes of the parent. This half-quantity is
called the haploid or N condition, while the full complement of genes in all of our other
cells, called somatic cells, is known as diploid or 2N. If a sex cell were not haploid, then
the genes in the sex cells would double with each successive generation.

Every species has a set number of chromosomes. A mosquito has six chromosomes
per cell; a sunflower, 34; a human, 46; a dog, 78; and a little goldfish, an impressive 94
chromosomes. In contrast, gametes of each of these species contain only half of these
numbers: a mosquito gamete has 3 chromosomes, a sunflower, 17; a human, 23; a dog,
39; and a goldfish, 47 chromosomes.

In a diploid cell, each chromosome has a partner, much like a pair of shoes (see Fig-
ure 6.7. The chromosome partners are known as a homologous pair. Homologs have one
maternal and one paternal copy of a chromosome. Alleles on each homologous chromo-
some code for the same trait. An allele for eye color, for example, on one chromosome
codes for eye color alongside the allele on its homologous pair. Figure 6.7 shows the

Figure 6.7 Homologous genes, knows as alleles, occur at the same location and
code for the same traits. From Biological Perspectives, 3rd ed by BSCS.

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Fertilization

Is the process in which
male and female sex
cells unite.

Zygote

A fertilized egg cell.

Haploid (N)

The half number of
chromosomes of the
parent.

Somatic cells

The full complement
of genes in all of other
cells.

Homologous

The chromosome
partners in a diploid
cell.

Diploid (2N)

The full complement
of chromosomes in all
body cells (except sex
cells).

Meiosis

A special form of
cell division in which
the newly produced
daughter cells contain
only half the number
of chromosomes of
the parent.

Gametes

Reproductive cells
(not given in bold
in text).

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202 Unit 2: Is it all in the Genes?

alleles on a chromosome (in varied colors). Before meiosis and mitosis take place, homol-
ogous chromosomes are duplicated. Thus, each replicated pair is composed of two sister
chromosomes, identical to each other. Each set of duplicated homologous chromosomes
contains four strands altogether: two original homologs and two duplicated strands.

Each homolog of the pair contributes one allele for a trait to its offspring. As shown
in Figure 6.8, homologous chromosomes separate into four gametes during production
of sex cells. Whether the individual homologue gets into a sperm or egg depends upon

Figure 6.8 Meiosis. Homologous chromosomes separate eventually into four sex cells (gametes). The doubling
of genetic material takes place before the parent cell is able to divide. From Biological Perspectives, 3rd ed by BSCS.

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Chapter 6: Inheriting Genes 203

(a1)

chance, as described by Mendel. During meiosis, homologous chromosomes separate
and move into one or the other of the gametes produced. They have an equal chance of
entering a newly formed gamete because chance determines their entrance. Homologous
chromosomes are inherited separately, as shown by Mendel’s law of segregation. Trace
the movement of replicated chromosomes in Figure 6.8 to find the gametes’ destination.

The diploid number (2N) of chromosomes in a parent cell is divided equally into
the sex cells during meiosis. Thus, the halving effect on the chromosome number occurs
during gamete formation. The result is a set of haploid (N) sex cells. This halving effect
counteracts fertilization, which unites genetic material from two sex cells into one
somatic cell, the zygote or fertilized egg cell. The result is a unification of N + N = 2N.

As demonstrated in Figure 6.9, meiosis follows a series of stages similar to those
seen in mitosis. Indeed, the names are also the same for the phases in both mitosis and
meiosis. There are a few differences:

1) In meiosis, there are two sets of the same series of stages, meiosis I and meiosis II;
but only one series in mitosis. This results in two cell divisions in meiosis and only
one cell division in mitosis.

2) In meiosis, four new daughter cells are produced as a result of the two divisions,
while only two are produced by mitosis.

3) Each daughter cell contains only the haploid number of chromosomes in meio-
sis, but daughters in mitosis contain the diploid.

4) Gametes contain a variety of genetic possibilities, in part because homologous
chromosomes separate into one or another of the sex cells, forming innumerable
combinations.

Figure 6.9 a. Phases of meiosis. There are two stages of meiotic cell division, I and II. The end result of
meiosis is the production of four haploid gametes (sex cells). Meiosis occurs in eight stages with descriptions
of each stage given in the figure. b. Mitosis occurs in one division and results in two identical cells.

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204 Unit 2: Is it all in the Genes?

(a2) ©
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Figure 6.9 (continued)

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Chapter 6: Inheriting Genes 205

the phases of Meiosis
In a period before meiosis, the interphase carries out functions similar to those during
the interphase before mitosis: cells grow in size, organelles duplicate and grow, and
genetic material doubles in the nucleus. When genetic material doubles during inter-
phase, two pairs of homologous chromosomes are formed. The purpose of meiosis is
to produce daughter cells capable of fertilization. To do this, fertilization requires two
haploid cells with haploid genetic material to unite.

During the first series of stages of meiosis, called meiosis I, homologous chromosomes
separate (refer Figure 6.9 to see each stage). Just like the first stage of mitosis, when a cell
begins meiosis, nuclear material condenses, transitioning from chromatin to chromosomes,
its nuclear envelope disappears, and chromosomes attach to a spindle fiber. Unlike mito-
sis, the first stage of meiosis I, called prophase I, homologous chromosomes in proximity
to each other exchange genetic material through a process called crossing over. In cross-
ing over, segments of one chromosome swap with segments of another pair. Crossing over
enhances the genetic combinations possible in gametes, as shown in Figures 6.8 and 6.10.
Areas that are crossed over randomly swap genetic material, leaving each homolog with a
unique set of DNA.

In metaphase I, the homologous chromosomes line up as pairs, which later separate
and move to opposite poles during anaphase I. Spindle fibers pull the pair of dupli-
cated homologs into the center. In the next phase, anaphase I, homologous chromosomes

Table 6.1 Comparison of meiosis and mitosis

Courtesy Peter Daempfle.

Meiosis Mitosis

Number of cells Four new cells produced Two new cells produced

Number of divisions Two cell divisions One cell division

Genetics of cells Haploid cells made Diploid cells made

Compared to parents and each other Different (variability) Identical

Figure 6.9 (continued)
Interphase
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis

(b) ©
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Metaphase I

The stage of mitosis
and meiosis that
follows the prophase
stage and precedes the
anaphase stage (not
given in bold in text).

Crossing over

The exchange of
genes between
chromosomes.

Prophase I

Also called the first
stage of meiosis I, in
which homologous
chromosomes in
proximity to each
other exchange
genetic material
through a process
called crossing over.

Meiosis I

The process of
cell division by
which homologous
chromosomes
separate and new cells
are haploid.

Anaphase I

The stage of cell
division in meiosis in
which homologous
chromosomes
separate.

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206 Unit 2: Is it all in the Genes?

separate to opposite ends of the cell. They are pulled apart in a random manner. A pater-
nal homolog may be pulled onto one side while a maternal homolog may be pulled onto
another side. At this point, the developing cells are haploid – with half the number of a
complete set of chromosomes. With 23 sister chromosomes pairs, there are 2n possible
new combinations. Thus, with 23 pairs of chromosomes in humans, there are 223 new
possible genetic combinations in each newly formed gamete: 2 × 2 × 2 × 2 . . . 23 times!
The genetic variation produced by random assortment is enormous.

Mendel hypothesized this random segregation of chromosomes, long before an
understanding of the phases of meiosis. Thus, three sources of genetic variation among
organisms are seen: 1) meiotic segregation of chromosomes; 2) random mutations in
genes as discussed in Chapter 5; and 3) crossing over, as discussed earlier in this section.
The processes of obtaining genetic variation are shown in Figure 6.10.

Figure 6.10 Genetic variation is introduced in species, especially during meiosis.
a. Crossing over. b. Mutation. Recombination and independent assortment during seg-
regation of alleles. All of these mechanisms add genetic diversity to cells and organisms.
From Biological Perspectives, 3rd ed by BSCS.

(a) (b) ©
20

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Chapter 6: Inheriting Genes 207

After this, telophase I and cytokinesis reform the nuclear envelop, with two new
daughter cells containing their own nucleus. These new cells are haploid or N, contain-
ing only half the original number of chromosomes. When homologs are pulled apart
during meiosis I, sister chromosomes are placed in daughter cells. The genetic compo-
sition of sister chromosomes is identical for the two. Thus, for these daughter cells, it is
like getting two left shoes instead of a right and left. The two daughter cells of meiosis I
are haploid, but contain a double set of half of the chromosomes.

Separating of sister chromosomes occurs during the next series of stages of meiosis,
called meiosis II. A short period separates meiosis I and II in a brief interphase. In this
time, there is no new duplication of genetic material and quickly cell division resumes
into prophase II. Chromatids reorganize, coiling tightly once again as chromosomes, in
preparation for the pulling apart process. During metaphase II, chromosomes line up
singly and then the two sister chromatids (which are identical In anaphase II, identi-
cal chromosomes separate, pulled apart by spindle fibers to opposite poles. In the last
phase, telophase I, nuclei reform and chromosomes become tightly coiled once again.
The physical separation of cytoplasm takes place during cytokinesis, as it pinches off to
become two new cells. The end result of telophase II and cytokinesis, in which two new
nuclei and cells form, is a total of four new haploid or N daughter cells.

As a result of meiosis, each human gamete contains only a haploid, 23 single strands
of chromosomes, much like having 23 “left” shoes. It is fertilization by another gamete,
containing 23 “right” shoes, that gives new life with a full diploid set of 23 pairs of chro-
mosomes. Figure 6.11 shows chromosomes during meiosis represented as shoes.

Male and Female Gametes
In animals, male meiosis produces four new sperm; and in females, one egg and three
polar bodies form. All four gametes are haploid as products of male and female meiosis.
Their nuclear material is evenly divided in both sexes. However, cytoplasm is unevenly
divided in females. During a female’s telophase I, most of the cytoplasm is retained in
one daughter cell, leaving the other three with very little cytoplasm.

Telophase I

The stage resulting in
the forming of a set of
new cells.

Meiosis II

The stage in which
sister chromosomes
are separated.

Metaphase II

The stage in which
chromosomes line up
singly and then the
two sister chromatids
separate and move
to opposite poles of
the cell.

Figure 6.11 Chromosome separations of “shoes.”

Telomere
Nucleus

Cell

Chromosome

Prophase II

The first stage of
meiosis II.

Cytokinesis

The division of cell
cytoplasm following
mitosis or meiosis.

Telophase II

The last stage in
the second meiotic
division of meiosis.

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208 Unit 2: Is it all in the Genes?

• Although note that not all animals reproduce sexually. For example, in ants, bees
and wasps, a virgin birth (parthenogenesis) takes place to produce males, which
will be discussed in Chapter 20.

During the next meiotic division, another unequal partition of cytoplasm happens. In
most female animals, the result is a set of three small sex cells and one large sex cell.
The daughter obtaining most of the cytoplasm becomes the female egg, while the others
become polar bodies. Polar bodies generally disintegrate quickly and are not viable for
fertilization. In human males, four gametes are made per meiotic division (Figure 6.12).
However, many divisions occur simultaneously, continuously producing large numbers
of gametes. The average ejaculation contains about 225 million sperm. In females,
there is generally only one egg in a cycle. A great deal of energy is placed into egg pro-
duction, but sperm are made en masse.

In most plant and animal species, the female gamete contains most of the cytoplasm.
Can you deduce why? The egg will provide most of the resources, both nutrients and
organelles, for a developing zygote. Once fertilized, an egg has the full complement
of genetic material from unification with a sperm. Its cytoplasm provides an excellent

Figure 6.12 Gamete development. a. Females who carry out oogenesis (egg formation)
and b. males, who carry out spermatogenesis (sperm formation). Both result in the pro-
duction of four haploid gametes, but males produce four sperm (in the tubules of the
testes) and females produce one viable egg with three polar bodies (within ovaries).

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Oogenesis

Mitosis
Meiosis I
Meiosis II

Primary egg

arrested in
prophase I

Primary egg

Secondary egg

Zygote
(a)

First polar
body (dies)

Second polar
body (dies)

(completed only
if fertilized)

Secondary egg, arrested
in metaphase II, ovulated

2n
2n
2n

n
n

Oogonium

Primordial
follicle

Primary
follicle

Growing
follicle

Mature
follicle

Ovulation

Corpus
luteum

Before birth

Childhood – ovary inactive

From puberty to menopause

Follicle development

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Chapter 6: Inheriting Genes 209

nutrient resource for the new organism’s survival. Also, by having instant organelles
on hand for growth and development, the new embryo has an advantage. This is why
mitochondria and chloroplasts are so beneficial to simply inherit, in accordance with the
endosymbiotic theory – instant energy and food production for a new organism.

sex: a Cost–Benefit analysis
Why sex? It has its advantages and its disadvantages for a species. Asexual reproduc-
tion is more efficient and requires less cell machinery. Prokaryotes reproduce by binary
fission, simply splitting in half to form two new organisms. The main disadvantage
of asexual reproduction is limited genetic variation. Asexual reproduction perpetuates
the genotypes of its parents, changing very little from generation to generation. Sexual
reproduction instead, leads to many varieties of offspring, enabling some organisms to
survive during changing conditions.

If, for example, a change in the environment should occur, as in the potato famine in
Ireland in the 1800s, all asexually produced offspring will respond in the same manner.
In Ireland, all of the potato plants at the time were grown asexually from the same origi-
nal plant. The organisms produced, with the same genetic variety, were susceptible to the
same fungal-like protist, causing them to decay and leading to famine. Genetic variation
allows for differences in a group so that at least some will survive. Variety in potatoes in
Ireland at the time would have saved over two million lives.

2n 2n

2n
n

n n n n

n
n
n
(b)

Spermatogenesis

Seminiferous tubule

Type A

spermatogonia

Type B

spermatogoniumType A

spermatogonium

Primary
spermatocyte

Secondary
spermatocytes

Spermatids
(2 stages of

differentiation)

Spermatozoa

Spermiogenesis

Meiosis II
Meiosis I
Mitosis

Lumen

Figure 6.12 (continued)

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210 Unit 2: Is it all in the Genes?

Sexual reproduction, on the other hand, allows new combinations of genes to form
in offspring. Through crossing over, segregation, and mutation, many genetic combi-
nations are possible. Of course, asexual reproduction allows for some variation due to
random mutations in organisms’ gene sequences during replication, but overall it results
in limited variety. Sexual reproduction gives a survival advantage in the process of
evolution – it provides enough genetic variation among individuals to help them adapt,
as a species, to environmental changes better than asexually reproducing organisms.

THe BeNeFITS oF Sex IS DeBATABLe

When observing the praying mantis’s mating ritual, in which a male has innate
fear of its mate, one still wonders if it is all worthwhile. One hypothesis con-
tends that a female bites his head off during copulation (the act of sex), in
order to “ease his mind” and relax during sex (Figure 6.13). This allows more
of his sperm to enter into her. She is not wasteful, and eats his whole body
after sex in order to gain energy for her developing embryos. Sex can be very
efficient in its quest to build a better species.

Figure 6.13 Praying mantis sex. Soon after copulation, she will bite off his
head and consume his body for the energy to raise their young.

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Determining sex
Upon closer inspection of the 23 pairs of chromosomes in humans, the final smallest
pair are the sex chromosomes. The other 22 pairs are called autosomal chromosomes,
which carry out a cells’ life functions. Human sex chromosomes are either X or Y chro-
mosomes, and these determine the sex of an organism. If a human has both an X and
Y chromosome, or is XY, it is male; and if it has two X chromosomes, or is XX, it is
female. The sex chromosomes differ from each other in a number of ways: a Y chromo-
some is much smaller than an X; a Y chromosome carries very little genetic information;
and a person can survive without a Y chromosome. After all, human females carry only
two X chromosomes. A karyotype, which shows a visual map of a set of chromosomes
for an organism, is given in Figure 6.14. In some disorders, chromosomes fail to separate

x chromosome

The final smallest
pair of the 23 pairs
of chromosomes in
humans.

Y chromosome

A sex chromosome
that is found only in
males (not given in
bold in text).

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Chapter 6: Inheriting Genes 211

and an abnormal number of chromosomes are seen. For example, in disorders such as
Down syndrome, an extra chromosome #21 is found after the failure of that chromo-
some to separate during meiosis.

In ants, bees, and wasps, in the order hymenoptera, queens produce haploid males
and diploid females, making females more related genetically to each other than to
males. In fact, they share 75% similarity in DNA, because females have all of the
same genes in common from their fathers. The father is haploid and has only one set
of the same chromosomes to give to all of his daughters. This phenomenon is known
as haplodiploidy, in which some offspring are haploid and some are diploid. This is a
basis for close relationships in ants, bees, and wasp societies: they share duties very
closely within colonies. In fact, most females within a colony give up sex altogether,
remaining sterile castes whose main purpose is to serve the queen master (Figure 6.15).
Many plants and all earthworms have both male and female parts; they produce both
male and female gametes. Sometimes simple temperature determines sex, as in turtles,
lizards, and reptiles. In turtles, cooler temperature eggs become males, while warmer
temperatures elicit females.

Figure 6.14 Human karyotype.

Human karyotype

1 2 3 4 5

19 20 21 22 Y X

14 15 16 17 18

7 8 9 10 11 12

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Figure 6.15 Worker ants helping their queen. Loyalty is strong for a queen who
controls all aspects of ant society. In this image, worker ants move their queen’s eggs,
serving both their queen and their future sisters who will hatch from those eggs.

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212 Unit 2: Is it all in the Genes?

Mendelian traits: single Gene Characteristics
Mendel did not yet know about molecular structures and the chemical idea of the gene
discussed in chapter 5, but his explanation for their transmission was remarkably accu-
rate for many traits: that there is pattern to their heredity and that they are inherited
as discrete units. Traits that are determined by instructions on a single gene are called
Mendelian characteristics, or single-gene traits. There are more than 9,000 single-gene
human traits that follow the principles of Mendelian genetics. These are either-or char-
acteristics: an organism has either one type or the other.

Mendelian
characteristic
(single-gene trait)

Traits that are
determined by
instructions on a
single gene.

Sex IS NoT SexuAL PReFeReNCe

Most research supports a strong genetic basis of sexuality. Behavioral genetics
is the research specialty that studies the genetic basis of behavior, including
sexual preferences. Sexual drive and desire vary across a continuum in most
animal societies from asexual (no sex) to hypersexual (excessive sex). It is
not a simple like or dislike of certain attributes in the opposite sex. Studies of
monozygotic (identical) twins show high contributions of genetic influences for
sexual preferences.

Biological bases for sexuality lie in two factors in animals: 1) activity of the
medial preoptic area of the brain (MPOA) and 2) DRD4 dopamine receptor
gene. Dopamine is a neurotransmitter found in the brain. Neurotransmitters
are chemicals that affect different parts of the brain. In humans and rats, for
example, the greater the activity in the MPOA area and the greater the number
of DRD4 receptors, the higher the sexuality rates in humans and rats.

A range in sexual drives and behaviors makes sense evolutionarily.
Hypersexuality, or having many sex partners, may appear favorable for enhancing
one’s reproductive success (more offspring with more partners), but this is not
so—quality also counts. Consider that after fertilization, in many animals a seminal
plug forms after a male ejaculates. If another partner enters, the plug is dislodged
and this next partner is also able to produce a viable offspring. In promiscuity, the
final partner is equally likely to father the child as compared with the first partner.
Usually the last partner in is weaker, older, and has poorer quality genes than the
first. In animal systems, hypersexuality is therefore selected against, with many
partners leading to weaker offspring. Experimental evidence shows that hyper-
sexual behavior in rats leads to decreased reproductive success for the female.

At the other extreme, asexuality, which is a lack of sex drive, is observed in
about 1% of humans. Why do such genes persist? One obvious answer is that
a lack of sexual attraction does not mean lack of sexual behavior.

On the other side, one would also expect homosexuality to be selected
against as it does not lead to new offspring. Another hypothesis as to why
“gay” genes remain in our gene pool is based on kin selection. Kin selection
is the theory that evolution favors helping between family members or kin to
augment the transmission of their related genes. People who do not have their
own children are more likely to help their nephews and nieces (kin), who are
25% identical to them. This behavior perpetuates their own genes more than
not having any children. Thus, there is strong evidence for a genetic basis of
sexual preference and helping behaviors.

Neurotransmitter

Are chemicals that
affect different parts
of the brain.

Hypersexuality

The condition in which
one has many sex
partners.

Asexuality

The lack of sex drive.

Kin selection

The theory that
evolution favors
helping between
family members or
kin to augment the
transmission of their
related genes.

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Chapter 6: Inheriting Genes 213

Figure 6.16 Examples of single-gene traits. A variety of characteristics are controlled by a single gene pair.
Tongue rolling, for example, is dominant over not being able to roll one’s tongue and attached ear lobes are
recessive. What Mendelian characteristics do you have? a. Colin Farrel, shown here with his sister, has a wid-
ow’s peak. b. This father and son both exhibit the tongue rolling ability. c. This man’s ear lobes are attached.

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Figure 6.17 Examples of traits in three patterns of inheritance: autosomal dominant, autosomal recessive, and
sex-linked traits. Each method of inheritance depends upon the expression of genes. Pedigrees for each pattern
of inheritance give affected and normal individuals in each generation. From Biological Perspectives, 3rd ed by BSCS.

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Consider being able to roll your tongue or not roll your tongue; having a widow’s
peak or not having a widow’s peak; and having albinism or not having albinism. Each
is determined by whether one has dominant or recessive sets of alleles. A person who
has a widow’s peak has a dominant allele dictating that the characteristic will show up.
Figure 6.16 illustrates a few single-gene traits.

There are three possible patterns of inheritance of single-gene traits leading to an
organism’s outward appearance: 1) autosomal dominant, in which the dominant allele
gets expressed, 2) autosomal recessive, in which both recessive alleles are present for
a person to get the recessive trait, and 3) sex-linked, in which the X chromosome deter-
mines the characteristic (Figure 6.17). Each pattern follows Mendel’s rules, expressing
the dominant allele in the phenotype. Examples of traits for each pattern are given in
Figure 6.17.

Autosomal Dominant. Diseases that are autosomal dominants are expressed when
even one allele is contained within a genotype. For example, in Huntington’s disease, a
degenerative and progressive muscular illness, the trait is inherited as a dominant allele.
If a person receives the autosomal dominant Huntington gene, she or he will develop its
related disease. Symptoms usually develop after an age of 30 years, well after she or he
could pass it onto children.

Singer Woody Guthrie, who sang “This land is Your Land,” died from the disease
at an age of 55 years, 13 years after symptoms appeared. He was the father of singer

Sex-linked

One of the three
possible patterns of
inheritance of single-
gene traits in which
the X chromosome
determines the
characteristic.

x chromosome

A sex chromosome
that is found twice in
females and singly in
males (not given in
bold in text).

Autosomal
dominant

The patterns of
inheritance of single-
gene traits in which
the dominant allele
gets expressed.

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214 Unit 2: Is it all in the Genes?

Arlo Guthrie, who did not inherit the disease from his father. Arlo had a 50:50 chance
of getting Huntington’s disease. Its origin is thought to have arisen from a small town in
Venezuela. About 30,000 Americans suffer from the disorder today.

Autosomal Recessive. Most diseases are carried on recessive alleles. Recessive
alleles stay hidden within a genotype without being expressed for longer periods of time
than autosomal dominants. As discussed earlier in this chapter, a person may harbor
a recessive allele without knowing it is present; the dominant allele covers its effects
within the genotype. Thus, deleterious recessives persist in groups.

For an autosomal recessive trait to be expressed, an individual must inherit one
recessive allele from each parent. Thus, two unaffected individuals have a 25% chance
of having an affected child. In Xeroderma pigmentosum, lack of DNA repair enzymes
due to recessive alleles leads to skin lesions and skin cancers

While certain forms of porphyria as described in our story are inherited in an auto-
somal dominant pattern (AIP), other forms occur through an autosomal recessive pattern
(congenital porphyria). In both forms, those affected lack enzymes to produce heme
groups in red blood cells. Because oxygen is carried throughout the body by heme
groups lack of heme causes damage to body systems. (Human systems will be discussed
in later chapters.) Both dominant and recessive porphyria are difficult to treat because
insufficient blood causes irreversible damage to vital organs. In January 2013, it was
reported that the remains of the mad King George III of England were discovered. His
mental health as a leader was in question throughout his reign. King George III likely
suffered from porphyria. His mental deterioration and decline are chronicled in the 1994
film, The Madness of King George. Many of the royal families married kin; increasing
chances for inheriting harmful genes, such as porphyria.

Sex-Linked. Sometimes males have a greater chance of inheriting a trait than females.
This occurs in sex-linked traits, in which a trait is determined by a gene located on a
sex chromosome, making inheritance patterns different between males and females. In
sex-linked traits, such as in color-blindness, often the disease-causing allele is recessive.
Most genes are found only on the X chromosome, so it determines the expression of a
trait.

If a female has one gene for color blindness, for example, she will not become color
blind if she has another dominant, normal gene on her other X chromosome. The dom-
inant allele masks the recessive allele causing color blindness. Alternatively, the same
situation in a male would result in color blindness. A male does not have two X chro-
mosomes to hide the one troublesome, recessive gene. Because a male has a Y chromo-
some, which has very little genetic information, it does not hide the effects of the normal
dominant allele. Sex-linked traits are more common in males than in females because
of this pattern (see Figure 6.18). Females have greater opportunity to hide alleles with
genes from their other X chromosome.

not so Mendelian Genetics
Most traits do not act as Mendel predicted. How do we explain why there is not simply
one or two possible skin colors? If all traits were Mendelian, all organisms of a species
would have either one phenotype or another, with no variations in between. Obviously,
this is not the case for most organisms’ characteristics. Other inheritance patterns pro-
duce the phenotypes most common to us: skin color, IQ, blood types, height, weight,
and sexual preference to name a few. While Mendel had great insights into his data,
most of our genetic expression is more complex than the seven pea plant traits he
chose to study.

Autosomal
recessive

The patterns of
inheritance of single-
gene traits in which
both recessive alleles
are present for a
person to get the
recessive trait.

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Chapter 6: Inheriting Genes 215

Incomplete Dominance
Incomplete dominance results from two different alleles contributing to gene expres-
sion. Snapdragon plants, for example, occur in red and white varieties, but may produce
pink flowers when mated together. A cross between a white and red Snapdragon plant
is shown in the Punnett square in Figure 6.18. The red and white alleles are equally
expressed in snapdragons, resulting in a pink color.

Multiple alleles

Some traits are controlled by several genes, each expressing a particular phenotype.
These traits are examples of multiple allelism. Individuals still carry only two of the
multiple alleles at any one time, one from a father and one from a mother. However,
the traits are all expressed within a population. In human blood groups, there are three
alleles controlling blood types: allele A, allele B, and allele O. Alleles A and B are
codominant, or share dominance with each other, and allele O is recessive. When allele
A or B are present with O, as in AO or BO, the result is a blood type of A or B, respec-
tively. When A and B are inherited together, a blood type AB results, and when allele O
is homozygous with OO as the genotype, the result is blood type O.

Alleles code for antigens, or special proteins on plasma membranes of red blood
cells: allele A codes for an A antigen, allele B codes for a B antigen, and allele O codes
for no antigen. Antibodies are chemicals made by the immune system that initiate an
attack on foreign bodies. When blood types with foreign antigens mix, antibodies are
made against antigens found on red blood cells.

Incomplete
dominance

A genetic situation
in which one allele
does not completely
dominate another
allele.

Multiple alleles

A series of three or
more alternative forms
of a gene, out of which
only two can exist
in a normal, diploid
individual.

Figure 6.18 a. Sex-linked traits. Inherited on the X chromosomes, they are more likely to appear
phenotypically in males. b. Snapdragons. Red and White Cross of F1 generation results in pink plants. 50% of
the offspring exhibit incomplete dominance, showing a pink coloration. This phenotype was not seen in its
parents. From Biological Perspectives, 3rd ed by BSCS.

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216 Unit 2: Is it all in the Genes?

To apply this, blood type O may be donated to any other blood group because it
contains no antigens on its red blood cells for which to attack. Blood type O-is therefore
called the universal donor. Blood type AB contains both A and B antigens on the red
blood cells. Therefore, a person with blood type AB is able to receive all other blood
types because they appear non-foreign to an AB immune system – all of the antigens are
already on its red blood cells. Blood type AB+ is therefore called the universal recipient.
Blood type A cannot donate to blood type B and vice versa. Blood type A has A antigens
and makes antibodies for B (because B appears foreign to it). Blood type B has B anti-
gens and makes antibodies for A (because A appears foreign to it).

Note that “+” and “−” have been used to describe blood types. Blood is classified
as either positive “+” or negative “−” because of a surface protein marker on red blood
cells, called the Rh factor. If blood contains an allele coding for the Rh marker, then its
blood is considered positive. Type A+ blood contains at least one allele for the A antigen
and one allele for the Rh factor. The Rh marker is another substance for immune cells to
recognize and attack. Those with Rh positive blood types are able to receive Rh negative
blood. Those with Rh negative blood, however, are not able to receive Rh positive blood.
Rh positive blood contains the Rh marker, which would be recognized and rejected by
immune cells of an Rh negative person. Figure 6.19 shows the four blood types along
with their antigens and the red blood cells associated with each blood type.

polygenic Inheritance
Most of an organism’s characteristics are polygenic traits, which are traits with patterns
of inheritance determined by more than one gene and influenced by the environment.
These include height, skin color, eye color, weight, hypertension, cancer, and heart dis-
ease. Polygenic traits are said to be continuous, with many levels expressed along a bell-
shaped curve. Figure 6.20 shows the curve for height in athletes as they have changed in
the past century. Both exhibit a polygenic bell shape, but the average has increased con-
siderably. What factors in society have changed to increase average height in our society?

Dominance or recessive expression is not so clear cut for polygenic traits. We are
not either short or tall, strong or weak, a smart or a bad student or even a brown or blue
eye color. There are many variations in between these extremes. Most individuals cluster
around an average with very few found at the extremes.

universal donor

A person of blood
type O who may
donate blood to any
other blood group
because the blood
group contains no
antigens on its red
blood cells.

Polygenic traits

Are traits with
patterns of inheritance
determined by more
than one gene and
influenced by the
environment.

Figure 6.19 Codominance and multiple alleles. a. There are four discrete blood types in humans: A, B, AB, and
O. Three different alleles determine blood type. Blood type is expressed as codominance with alleles sharing a
phenotypic expression. b. Genetics of the human ABO blood groups. From Biological Perspectives, 3rd ed by BSCS.

Antigen A

Anti-B Antibody

(a) (b)
Type A

Red Blood
Cells

Plasma

Anti-B Antibody Anti-A Antibody

Type B

Neither Anti-A nor
Anti-B Antibodies

Type AB

Anti-A and Anti-B
Antibodies

Type O

Antigen B Antigens A and B Neither Antigen
A nor B

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universal recipient

A person of blood
type AB who may
receive blood from
any other blood group
because the blood
group contains all
antigens on its red
blood cells.

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Chapter 6: Inheriting Genes 217

Polygenic traits are influenced by the environment because genes alone do not explain
the variation in phenotypes. They are called multifactorial traits because they have many
factors that affect their expression. Environment interacts with genes to form a phenotype.
Obesity, a polygenic trait, was studied to determine the effects of genes and the environment
on its expression in humans and mice. Identical twins, which have the exact same geno-
types because they arise from the same fertilized egg, were studied. Twin studies often mea-
sure how much a polygenic trait is due to genetics. Obesity had a concordance rate of 70%,
meaning that 70% of the time obesity is found in both twins, regardless of what they ate.

The mouse Ob gene encodes for a weight-controlling hormone, leptin, produced in
fat cells. Figure 6.21 shows two mice, one overweight, with a mutated Ob gene, and one
normal weight, with a normal Ob gene. The human gene for leptin is on chromosome
#7 and its mutation increases the risk for developing obesity. However, a mutation of
the leptin gene is not the only contributor to obesity. Obesity is a complex disorder,
involving the interaction of several genes with the environment. Indeed, scientists have
detected genes for obesity in humans on chromosomes: #2, #3, #5, #6, #7, #10, #11, #17,
and #20. Research on this multifactorial condition continues.

Figure 6.20 People are often categorized by their height. The mean height of men
today is 5’10”, whereas in 1913 it was 5’8″. The photo from 1913 shows a group of
college students categorized by height. Note that the categories follow a bell-shaped
curve, a characteristic of polygenic traits. What factors do you think contributed to the
change in average height over the past century?

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Figure 6.21 (a) Normal vs. (b) chubby rat, the ob gene has its effects on weight in rats (normal rat on the
left and obese rat on the right.)

(a) (b)©
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218 Unit 2: Is it all in the Genes?

In our story, porphyria symptoms emerge from genetic and environmental factors.
While there is a genetic component, stress, smoking, alcohol, and sun exposure trigger
symptoms of porphyria. It is also shown that garlic aggravates porphyria symptoms,
possibly the root of the assertion that garlic keeps vampires away.

There are eight enzymes involved in heme biosynthesis. Each enzyme has genes
that code for it. If any one of these genes is mutated, abnormal heme production results.
Thus, the disease has genetic roots as well as environmental triggers. It is multifactorial
because many (or multiple) factors affect its expression.

Some polygenic traits are due to gene–gene interactions with very little environmen-
tal input. For example, eye color is influenced by about 16 different genes, with less than
1% of its phenotype due to the environment (Figure 6.22). You may have assumed that
eye color is an either/or scenario, but in fact it is a polygenic trait, with a continuum of
colors possible. Have you ever wondered how hazel or green eyes develop? It is a matter
of pigments. The more genes inherited for pigmentation in the eye’s iris, the darker the
coloration. If there are no alleles for pigment production in one’s genotype, eyes will be
blue; if there is one or two genes, eye color will be green; if there are three or four alleles
for pigment, coloration will be hazel, and more alleles for pigment give varying shades
of brown.

pleiotropy
When one gene affects more than one trait, this effect is called pleiotropy. Several spe-
cies of farm birds – chickens, turkeys. – exhibit a “frizzle” mutation on one of their
genes. The frizzle allele causes bird feathers to be stringy and weak, providing poor
insulation. More seriously, the mutated frizzle allele affects the bird’s heart, kidneys, and
thyroid and impairs its overall health. Pleiotropy is seen in many characteristics from
phenlyketonuria (PKU) in humans, with effects on brain and skin functions, to multiple
congenital deformities in rats. All of the associated features of the disorders are due to a
single-gene effect on multiple traits.

tracing Gene Flow in Families: pedigree analysis
Pedigrees are diagrams of genetic relationships among family members through dif-
ferent generations; they are used to trace gene flow through a family (see Figure 6.23
as an example). They show patterns and help figure out whether one has a dominant or

Pleiotropy

The condition in which
one gene affects more
than one trait.

Figure 6.22 Eye color genotypes and phenotypes. Eye color is mostly written in our genes.

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Chapter 6: Inheriting Genes 219

recessive allele, based on one’s parents. The pedigree diagram uses circles to indicate
females and squares for males. No shading indicates unaffected individuals, shaded
are affected and half shaded are known carriers or heterozygotes. Horizontal lines
between circles and squares show mating and vertical lines show descent. Several
traits shown in Figure 6.23 indicate how genes are expressed through generations:
pedigrees for hemophilia and a family with porphyria are given.

tracing Gene Flow in Groups:
population Genetics
How do genes move between villages, cities, and continents? If you compare groups that
are separated geographically do you find different characteristics? Is there such a con-
cept as “race,” genetically separating different groups of humans? These questions have
answers in the branch of genetics called population genetics. A population is defined as
a group of individuals able to breed with each other in a given area, producing fertile off-
spring. The study of patterns of gene flow from one group to another and within groups
is known as population genetics.

Among other things, population geneticists investigate how diseases are carried in a
population of organisms. Mathematical calculations determine the frequency of alleles
in a group. These numbers help determine trends in gene flow over time. Porphyria was
found to be in high proportions in populations in the old Austro-Hungarian Empire’s
province of Transylvania, where the myth of vampirism originated in our opening story.
Further studies are being done to determine the exact origins. In the example of Hun-
tington’s disease, however, population geneticists determined that the gene arose from
one woman in a small town in Venezuela, according to records dating back to the 1700s.
Scientists collected information from 90,000 people and developed pedigrees to chart
gene flow. They tested blood samples to detect the disease and plotted its movement
through the years. Though Huntington’s disease is inherited, a 2001 study indicated that
roughly 10% of cases result from new, random mutations.

Understanding how genes move within a population can help explain why certain
genes persist in that population, and this in turn enables us to better understand diseases

Pedigree

Are diagrams of
genetic relationships
among family members
through different
generations; they are
used to trace gene
flow through a family
(not given in bold in
text).

Population genetics

The study of patterns
of gene flow from one
group to another and
within groups.

Figure 6.23 A pedigree shows the genetics of a family tree. a. Symbols are used to create the family tree.
This pedigree shows the inheritance of hemophilia by the royal families of Europe. Hemophilia is a sex-linked
trait. The Bettmann Archive. b. A pedigree of a family with congenital porphyria, a recessively linked trait.
From Biological Perspectives, 3rd ed by BSCS

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220 Unit 2: Is it all in the Genes?

and organism characteristics. For example, by mapping out where cystic fibrosis is
located geographically, scientists determined its benefits to immunity against cholera.

It is difficult to determine the exact number of carriers in a population because car-
riers exhibit a normal phenotype. However, scientists may use a mathematical formula to
estimate the probability of occurrence of a recessive allele in a population. The Hardy–
Weinberg quadratic equation for equilibrium shows the relative proportion of alleles in
a population through counting the number of recessive individuals:

p2 + 2pq + q2 = 1

In the equation, p equals the proportion of dominant alleles in a population and q equals
the proportion of recessive alleles within a population. Homozygous dominant organ-
isms are given as p2 and homozygous recessive are given as q2. Heterozygotes or carriers
are given as pq. Through counting the number of dominant individuals, which one is
able to detect through observation, p is calculated. Then, q is solved for, and the rest
of the equation’s letters are calculated using the quadratic equation. This is a quadratic
equation set equal to one. It assumes that a population is not evolving or changing in its
allele frequency. It assumes no immigration, emigration, natural selection, or mutations
that alter normal gene frequency.

Obtaining data through use of the Hardy–Weinberg equation helps determine the
risk of having a particular gene within one’s population, helps understand if a popula-
tion, such as a stand of red maple trees, is undergoing a change in gene flow, and exam-
ines how populations compare with each other based on genetic factors. For example,
with respect to the alleles for sickle-cell anemia: African American populations with
West African ancestry have a 12.5% prevalence of sickle-cell anemia, but West Afri-
can populations have a 20–40% prevalence. This indicates that the populations have
diverged in their overall genetic compositions.

Genealogy is the study of family history. It is related to population genetics, using
pedigrees to investigate one’s family history. New tests are available that allow one
to send in a blood or saliva sample and have it analyzed to trace genetic origins. For
instance, tests identify over 400 different ethnics groups in Africa from which our genes
may be compared to determine origins. Is this useful or does this further divide people
based on the social construct of race?

INBReeDING: Too CLoSe FoR CoMFoRT oR A GooD
STRATeGY?

Consanguinity, or sharing blood through mating with close relatives, such as broth-
ers and sisters, has been shunned by most societies throughout history. The cul-
tural taboo has a practical origin: inbreeding depression, or the loss of heterozygotes
and at the same time, the acceleration in the number of recessive alleles in a pop-
ulation that are often harmful. The Hardy–Weinberg equation shows the increase
of both recessives and their related diseases in studies of inbreeding groups.

Individuals in the same family share many genes in common. The recessive
genes that would otherwise be covered up by the dominant allele are more

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Chapter 6: Inheriting Genes 221

likely to become expressed when recessives occur more frequently. It is likely
that pockets of porphyria existed in Medieval Europe, where intermarriage
was somewhat common. Porphyria would have been more pronounced in such
areas, where dominant normal alleles for heme formation were less prevalent.
When close relatives mate, both are part of a lineage that has the potential
of sharing more of the same harmful genes in common. Examples may include
sickle-cell anemia, cystic fibrosis, or even cancer.

On the other hand, recent research shows that a certain amount of
inbreeding can produce healthier children. In a study of Iceland’s family history
lineage, marriage between third and fourth cousins produced the most numer-
ous and healthiest children over the past 1,000 years. It is hypothesized that
outbreeding, or mating with someone too different from one’s own genotype,
may also lead to health problems in children. In fact, about 20% of marriages
worldwide occur between first cousins. This practice is illegal in many of the
United States.

Outbreeding too far also has negative consequences, though. One such
example occurs for the Rh factor, cited earlier in the chapter. Rh is a set of
protein markers on red blood cells that need to match between mother and
child for a healthy baby to be born. If the mother is Rh negative, and the father
is Rh positive, then the blood of the second fetus who is Rh positive (from
the father) will be recognized as an invader by the maternal immune system.
Presently, Rhogam is a treatment given to pregnant mothers to prevent mis-
matched blood from causing a problem (Figure 6.24). Without modern tech-
nology, however, such a match would be disfavored. Thus, there is an optimal
level of inbreeding for reproductive success. However, third and fourth cousins
have only about 1/256 to 1/512 genes in common with one another, so the
chances of revealing recessive alleles is quite low.

Figure 6.24 Rhogam is used to treat Rh incompatibility between mother and fetal
blood types.

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222 Unit 2: Is it all in the Genes?

Gene technology: solving
problems Using Genetics
Biotechnology is the branch of science that uses biological knowledge and processes to
produce goods and services for human use and financial profit. Its techniques manip-
ulate genetic sequences in organisms to produce medical drugs and develop weather-
and pest-resistant crops, to name a few examples. One significant sub-branch is gene
technology, which modifies plants, bacteria, and animals to create products for society.
First, the genome of a specific organism is modified by inserting a gene from another
organism into the subject organism’s already existing DNA. The resulting organism is
called a genetically modified organism (GMO) and it is classified as transgenic because
it contains genes from another species. Inserted genes produce proteins, for which the
inserted gene codes. Human proteins such as insulin, to help diabetics, human growth
hormone or HGH, to help in dwarfism, and factor VIII to help hemophiliacs are pro-
duced by these GMO organisms. Transgenic tobacco plants produce HGH, as shown in
Figure 6.25.

Before gene technology, the available means of collecting these proteins had many
drawbacks. HGH was collected from dead bodies and could cause disease when injected
into patients, for example. Hemophiliacs, who suffer from life-threatening blood loss
due to the lack of a blood clotting factor, were dependent on blood transfusions, which
carry a risk of containing infected blood. Before AIDS was discovered in the early 1980s,
many hemophiliacs were infected with HIV from blood transfusions. Gene technology
changed their treatment options, leading to less risk. Hemophilia is now treated with
genetically produced clotting factor VIII. Lessened risk from disease-causing agents is
a great step forward for society due to gene technology.

GMOs are produced through genetic engineering, which is the manipulation
of an organism’s genes in a way other than is natural. This manipulation is accom-
plished through using a technique called recombinant DNA technology (Figure 6.26).
Recombinant DNA technology is the process by which DNA is extracted from nuclei of
organisms and treated with restriction enzymes. Restriction enzymes cut DNA at spe-
cific sequences. A bacterial plasmid, which is a circular strand of DNA, is also cut with

Figure 6.25 Transgenic tobacco plants. These plants are being used to produce
human growth hormone (HGH) to treat human growth disorders. A gene has been
inserted into these plants to produce HGH.

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iy
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to
c
k.
c
o
m

Gene technology

The technology that
modifies plants, bacteria
and animals to create
products for society.

Genetically
modified organism

Are organisms in which
DNA is genetically
altered via genetic
engineering techniques.

Recombinant DNA
technology

The process by which
DNA is extracted from
nuclei of organisms
and treated with
restriction enzymes.

Biotechnology

The branch of
science that uses
biological knowledge
and procedures to
produce goods and
services for human use
and financial profit.

Genetic
engineering

The process in which
an organism’s genes are
manipulated in a way
other than is natural.

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Chapter 6: Inheriting Genes 223

Figure 6.26 Genetic recombination techniques. They are steps used in producing
a genetically modified, transgenic organism. Note that the restriction enzymes cut
DNA at specific locations, allowing plasmid DNA to attach and become a “part” of the
DNA of the newly created transgenic organism. In this figure, the clotting factor VIII
gene is inserted into bacteria in order to produce factor VIII en masse for human use.
The bacteria made by genetic recombination are genetically engineered “transgenic”
organisms. From Biological Perspectives, 3rd ed by BSCS.

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224 Unit 2: Is it all in the Genes?

the same restriction enzyme. Bacterial and human DNA fragments are mixed together,
causing them to link with each other. The bacterial plasmid now contains the human
gene that will be used for coding new proteins. The plasmid is then transferred into
a new bacterial cell. This bacterial cell expresses the newly inserted gene to make the
desired protein. It divides over and over, forming new cells that make the a product. The
bacterium with its newly inserted gene is said to have been recombined.

In our story, Herbie might benefit if biotechnology treatment options were available
for porphyria. To date, porphyria is treated with limited success, with symptoms and
long-term problems plaguing its sufferers. An area of study that holds promise for more
successful treatment of porphyria and other diseases is gene therapy. Gene therapy is
the insertion of genes into an organism to treat its disease. In the past two decades, gene
therapy has had mixed success. Future research may find a way to insert a gene into por-
phyria patients such as Herbie, that blocks the mutated gene, which is unable to produce
normal heme groups. Another advance for porphyria sufferers would be in the area of
blood production. Presently, blood transfusions restore deficient heme in the blood of
porphyria sufferers. Panhemin is also a drug used today to treat porphyria by limiting the
liver’s production of porphyrins (Figure 6.27). Both treatments are derived from human
blood and have risks of carrying infectious agents.

Gene therapy

The process in which
genes are inserted into
an organism to treat
its disease.

Figure 6.27 Panhemin Vial.

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ARe PRoDuCTS oF BIoTeCHNoLoGY HeLPFuL
oR HARMFuL To SoCIeTY?

Many products are made available through the use of biotechnology. Trans-
genic crops, for example have greater resistance to herbicides, and viral and
fungal diseases. They are modified to withstand cooler temperatures longer
and grow faster with larger fruits and vegetables. Soybeans, corn, cottonseed,
and canola crops have seen large increases in transgenic numbers in the past
decade, as shown in Figure 6.28. Over 93% of all soybeans and cotton crops
are genetically modified in the United States. Eighty six percent of all corn, a
major staple for cattle and humans, is produced by GMO organisms. If these
organisms were not permitted to contribute to our food supply, would we be
able to sustain our need to produce food, as a world population?

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Chapter 6: Inheriting Genes 225

There have been big increases in farm production since the development of
GMO foods. Crops are hardier and more productive, but it is a hotly debated
area of study. The greater abundance of food means that fewer people go
hungry. However, some GMO foods may also be linked to disease. A 2012
study in Europe shows that a corn variety, NK603 containing genes making it
more resistant to the weed killer Roundup, was shown linked to cancer-caus-
ing effects when fed to a group of mice. Owing to this “cancer corn,” some
European nations are placing restrictions on transgenic products. Is this fear of
NK603 corn justified?

The public has been consuming GMO products for over 15 years. No
known ill effects have been confirmed by the scientific community in this time.
What effects will be shown in 10, 20, or 30 years from now, is yet to be
determined? Long-term results are not available because GMOs have not been
around long enough.

Figure 6.28 Graph showing relative increase in transgenic crops over the past 20 years.

0 20 40 60 80 100 120 140 160 180

1996
1997
1998
1999
2000
2001

2003
2002

2004
2005
2006
2007
2008
2009
2010
2011
2012

the things We’ve handed Down: should
We tamper With our Genes?
HGH produced by gene manipulation for the past 25 years helps extreme cases of growth
disorders. Before recombinant DNA techniques were available, HGH was extracted
from the pituitary glands of cadavers and carried the risk of contaminating patients.
HGH is now fast and easy to produce, without contamination risks, making it more
commercially available.

This brings ethical and practical medical questions into play: Should a preteen male,
predicted to grow to a height of about 5′ 4″, take the drug? What if it is against the doc-
tor’s advice, which is based on the American Medical Associations guidelines to restrict
the drug only for extreme cases? What is an extreme case? What are the side effects?
These are difficult questions to answer.

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226 Unit 2: Is it all in the Genes?

HGH has known side effects – from abnormal growth of joints to chronic pain, dis-
figurement and even death. HGH is used more and more in society to help teens reach
a desired height. Some people are very happy with the results and other are devastated.
There is uncertainty in medical procedures and treatments and their risks and benefits
should be weighed.

What are the social issues involved in being short? How many female readers would
date a person taller than they are? I presume many would. How many would date a per-
son shorter than they are? I am not sure. How many male readers would date a person
shorter than they are? I presume most. How many male readers would date a person
taller than they are? I am not sure.

The reader should weigh the pros and cons of using technologies that scientists have
made available to them. It is difficult to judge one another’s decision without under-
standing the social implications of the medical treatment.

Ethically, will another doctor help the patient if one doctor denies treatment? What
is ethical for doctors to do if a patient desperately wants treatment to grow taller? These
are just a few of the provocative questions about HGH that some young Americans face
every day.

MYTH, FABLe, AND SuPeRSTITIoN HAve exPLAINeD
MALADIeS INCoRReCTLY

In the story at the beginning of the chapter, the character ponders his iden-
tity as a vampire. Throughout history, unexplainable illnesses have often been
linked with folklore of the occult, witchcraft, and creepy creatures. Epidemics
of the plague, consumption (tuberculosis), and the like often led to exhuming
bodies and labeling one or more victims to some sort of myth or fable based
upon fear. All of these illnesses had biological origins, as shown in our opening
story, which shows the power of myth in shaping a person’s mental health and
outlook—Was the narrator in the story a victim of his own superstitions?

summary
Heredity is the study of inheritance of characteristics from parent to offspring. Predicted
patterns of inheritance were discovered by Gregor Mendel in the 1800s. Mendel’s three
laws describe inheritance of over 9,000 human traits. Inheritance of genetic informa-
tion is more complex than Mendel hypothesized. Genes interact with each other, the
environment, and sometimes share in their expression. Sexual reproduction results in a
great deal of variation in populations. Meiosis, the forming of sex cells, produces unlim-
ited genetic combinations within gametes. The flow of genes from one group to another
is studied by population genetics. The numbers of different genotypes and phenotypes
in a population are given by using the Hardy–Weinberg equation, with certain assump-
tions accepted. Biotechnology’s important component, gene technology, has resulted in
many products available for public use. Gene technology products are continually being
developed. Their effects on society and science continue to be debated as well.

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Chapter 6: Inheriting Genes 227

ChECk oUt

summary: key points

• Heredity affects our physical characteristics, our environment and our future generations.
• The discovery of inheritance by Gregor Mendel explains many of life’s characteristics.
• Inheritance can be explained in future generations by probability using Punnett squares and in

populations using the Hardy–Weinberg equation.
• The stages and products of meiosis explain how sexual reproduction leads to great genetic variation.
• Many traits in organisms are non-Mendelian, explained by codominance, polygenic inheritance,

multiple alleles, and pleiotropy.
• Pedigrees clarify gene flow within families.
• Population genetics studies gene flow between and within populations.
• Biotechnology has advances to provide products for human use, with debatable effects.

alleles
anaphase I, II
asexuality
autosomal dominant
autosomal recessive
biotechnology
crossing over
cytokinesis
diploid (2N)
dominant
fertilization
gametes
gene technology
gene therapy
genetic engineering
genetically modified organism (GMO)
haploid (N)
heredity
heterozygous
homologous
homozygous
hypersexuality
incomplete dominance
kin selection
law of dominance
law of independent assortment

law of segregation
meiosis, meiosis I, meiosis II
Mendelian characteristic, single-gene
trait
metaphase I, II
monohybrid cross
multiple alleles
neurotransmitter
pedigree
pleiotropy
polygenic traits
population genetics
porphyria
prophase I, II
recessive
recombinant DNA technology
sex chromosome
sex-linked
somatic cells
telophase I, II
testcross
universal donor
universal recipient
X chromosome
Y chromosome
zygote

KeY TeRMS

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Chapter 6: Inheriting Genes 229

Multiple Choice Questions

1. Which is an inherited disorder?
a. porphyria
b. obesity
c. Huntington’s disease
d. all of the above

2. Which of Mendel’s laws was derived from the presence of 100% yellow phenotypes
in the F1 generation?
a. law of dominance
b. law of independent assortment
c. law of continuity
d. law of segregation

3. The way in which an organism appears is its
a. genotype
b. phenotype
c. pleiotropy
d. codominance

4. If two heterozygous parents mate both carriers for a recessively inherited form of
porphyria) what is the chance that their offspring will have porphyria?
a. 0
b. 25
c. 50
d. 100

5. Which stage of meiosis involves the separation of homologous chromosomes?
a. anaphase I
b. anaphase II
c. prophase I
d. prophase II

6. Which represents the correct flow of stages in meiosis?
a. prophase II➔metaphase I➔anaphase I➔telophase I
b. prophase I➔metaphase I➔anaphase I➔telophase I
c. anaphase II➔prophase II➔telophase II➔metaphase II
d. telophase I➔anaphase I➔metaphase I➔prophase I

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230 Unit 2: Is it all in the Genes?

(a) (b)

7. Which is the source of hemophilia for Prince Frederick using the pedigree in the
figure below?
a. grandmother
b. grandfather
c. mother
d. uncle

8. In the Hardy–Weinberg equation, if the frequency of recessive alleles is 5% of the
population, what is the number of recessive individuals in that population?
a. 25 out of 10
b. 25 out of 100
c. 25 out of 1,000
d. 25 out of 10,000

9. In question #8 above, what is the frequency of dominant genes in the population?
a. 5%
b. 25%
c. 75%
d. 95%

10. Which statement best describes the benefits of GMOs to society?
a. Photosynthesis decreases greenhouse gas effects.
b. The food supply can support the population.
c. Nonnative species are kept in check.
d. GMOs kill many species of insects.

short answers

1. Describe how porphyria affects the health of those inheriting it. Describe the
mechanism by which porphyria causes damage. How does porphyria get portrayed
as vampirism in history? Is it justified? Why or why not?

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Chapter 6: Inheriting Genes 231

2. Define the following terms: phenotype, genotype, and pleiotropy. List one way each
of the terms differ from each other in relation to heredity. Give an example found
within fowl to make this clarification.

3. Describe the experiments of Gregor Mendel leading to the law of independent
assortment. How does this law relate to genetic diversity within offspring?

4. In question #3, describe two other mechanisms by which genetic diversity is
increased in populations through sexual reproduction.

5. Draw a Punnett square for a cross between two heterozygous tongue rollers? What
percentage of their offspring are heterozygotes? Homozygous dominant?

6. List the stages of meiosis I and II, indicating the point at which a cell becomes
haploid. Why does it become haploid at this point?

7. Describe a testcross used to determine genotype in a pedigree. What is an advantage of a
testcross to determine genotype in a pedigree? Are its results certain? Why or why not?

8. One in 22 people in the United States are carriers for cystic fibrosis. What is the per-
centage of individuals who actually have this disease, using the Hardy–Weinberg equa-
tion? Show your work.

9. Describe the process of recombinant DNA technology. Use the following terms to write
its description: restriction enzyme, bacterial plasmid, vector, human DNA, protein.

10. Define the process of inbreeding. What are disadvantages of inbreeding? Are there
any advantages? Explain your answers genetically.

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232 Unit 2: Is it all in the Genes?

Biology and society Corner: Discussion Questions
1. Diseases such a porphyria manifest in ways that give them a bad reputation. What

other inherited traits have a bad reputation in society? Choose one trait and discuss
how it is treated by the dominant culture. How are people with the trait treated
differently? Suggest ways to improve the lives of people with this trait within our
society.

2. The genetic basis of sexual preference was advocated for in this chapter. With which
side do you agree, genetic or environmental in cause? What factors do you think
limit or enhance the acceptance of alternative sexual preferences in society? Does
the idea of a genetic basis have an impact in this acceptance?

3. If a society decided to remove all of the harmful recessive genes, such as cystic
fibrosis, within its population, what would be its ethical difficulty? What would be
its practical difficulty, based on the Hardy–Weinberg equation? Explain you answer
fully.

4. Race is used in decision-making regularly in the U.S. organizations. Why is race
such an important factor in society? Do you think it should be so? Is there a genetic
basis to human race classifications?

5. A health food guru claims that GMOs are making people fat. Explain why this state-
ment is false. How have GMO foods helped society? How have GMO foods harmed
society? Is your answer certain? Why or why not?

Figure – Concept Map of Chapter 6 Big Ideas

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Unit 2: Is it all in the Genes?��

Chapter 5: Molecular Genetics��
Chapter 6: Inheriting Genes��

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