MOD 10 Genetics and Patterns of Inheritance

MOD 10 Genetics and Patterns of Inheritance

Using the book or Reading Essentials, define the following vocab words from Module 10: genetics, allele, dominant, recessive, homozygous, heterozygous, phenotype, genotype, law of segregation, hybrid, law of independent assortment, genetic recombination, polyploidy, selective breeding, inbreeding, test cross, carrier, pedigree, incomplete dominance, codominance, multiple alleles, epistasis, sex-linked trait, polygenic trait.

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Reading Essentials • Introduction to Genetics and Patterns of Inheritance
Introduction to Genetics and Patterns of
1 Mendelian Genetics
Think about what you have learned about the scientific method.
On the lines below, list some of the steps Mendel might have used
to learn about the natural world. In this lesson, you will learn
about Gregor Mendel’s experiments.
How Genetics Began
Gregor Mendel, an Austrian Monk, lived in the 1800s. He
experimented with pea plants in the monastery gardens.
Pea plants usually reproduce by self-fertilization. This means
that the female gamete is fertilized by a male gamete in the same
flower. Mendel discovered a way to cross-pollinate peas by hand.
He removed the male gametes from a flower. He then fertilized
the flower with the male gamete from a different flower.
Through these experiments, Mendel made several hypotheses
about how traits are inherited. In 1866, he published his findings.
That year marks the beginning of the science of genetics, the
science of heredity. Mendel is called the father of genetics.
The Inheritance of Traits
Mendel used true-breeding pea plants—plants whose traits
stayed the same from generation to generation. Mendel studied
seven traits—flower color, seed color, seed pod color, seed shape,
seed pod shape, stem length, and flower position.
As you read this lesson,
highlight any parts you do not
understand. After you have
read the lesson, reread the
parts you have highlighted.
Get It?
1. Infer why it is important that
Mendel’s experiments used
a true-breeding plant.
• the law of segregation and
the law of independent
• how to use a Punnett square
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Reading Essentials • Introduction to Genetics and Patterns of Inheritance
What did Mendel find when he crossed pea plants
with different traits?
Mendel called the original plants the parent, or P, generation.
The offspring were called the F1 generation. The offspring of the
F1 plants were called the F2 generation.
In one experiment, Mendel crossed yellow-seeded and greenseeded plants. All the F1 offspring had yellow seeds. The greenseed trait seemed to disappear.
Mendel allowed the F1 plants to self-fertilize. He planted
thousands of seeds from these plants. He saw that in these
offspring, the F2 generation, three-fourths of the plants had
yellow seeds and one-fourth had green seeds, a 3:1 ratio.
Mendel performed similar experiments for other traits. Each
time, he observed the same 3:1 ratio.
How did Mendel explain his results?
Mendel proposed that there were two forms of each trait, and
each form was controlled by a factor, which is now called an
allele. An allele (uh LEEL) is a different form of a gene passed
from generation to generation. Yellow-seed plants have a different
allele than green-seed plants.
Mendel proposed that each trait was controlled by two alleles.
The dominant form is the version of the trait that appears in the
F1 generation. The recessive form is the version that is hidden in
the F1 generation.
2. Label Fill in the boxes with
the name of each
generation of offspring.
Draw the peas you would
expect to see in the empty
pods. Use shading to
indicate a green pea.
3. Apply In Mendel’s
experiment with green and
yellow seeds, what was the
dominant trait?
green-seed plant yellow-seed plant
all yellow seeds
3/4 yellow seeds
1/4 green seeds
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Reading Essentials • Introduction to Genetics and Patterns of Inheritance
How does dominance work?
When written, the dominant allele is represented by a capital
letter. The recessive allele is represented by a lowercase letter.
An organism is homozygous (hoh muh ZI gus) if both alleles
for a trait are the same. The organism is heterozygous (heh tuh
roh ZY gus) if the alleles for a trait are different. In heterozygous
organisms, only the dominant trait can be seen. Dominant alleles
mask recessive alleles.
How do genotype and phenotype differ?
It is not always possible to know what alleles are present just by
looking at an organism. A yellow-seed plant could be
homozygous (YY) or heterozygous (Yy). An organism’s allele pairs
are called its genotype (JEE nuh tipe). The expression of an allele
pair, or the way an organism looks or behaves, is called its
phenotype (FEE nuh tipe).
What is the law of segregation?
Recall that the chromosome number is divided in half during
meiosis. The gametes contain only one of the alleles. Mendel’s
law of segregation states that the two alleles for each trait
separate from each other during meiosis and then unite during
fertilization. When parents with different forms of a trait are
crossed, the offspring are heterozygous organisms known as
hybrids (HI brudz).
A cross which involves hybrids for a single trait is called a
monohybrid cross. Mono means one. The offspring of the cross
have a phenotypic ratio of 3:1.
How are two or more traits inherited?
Mendel also performed dihybrid crosses, crossing plants that
expressed two different traits. Mendel crossed yellow, round-seed
plants with green, wrinkle-seed plants. Round seeds are
dominant to wrinkled, just as yellow color is dominant to green.
He wondered whether the two traits would be inherited together
or separately. Members of the F1 generation are dihybrids because
they are heterozygous for both traits.
Mendel found that the traits were inherited independently.
Members of the F2 generation had the phenotypic ratio of
9:3:3:1—9 yellow round seeds, 3 green round, 3 yellow wrinkled,
and 1 green wrinkled. From experiments with dihybrid crosses,
Mendel developed the law of independent assortment, which
states that alleles distribute randomly when gametes are made.
4. Predict What would be the
phenotype of a
homozygous, recessive (yy)
pea plant?
Get It?
7. Evaluate How can the
random distribution of
alleles result in a
predictable ratio?
Get It?
6. Restate Mendel’s law of
segregation in your own
Get It?
5. Infer whether an individual
with a recessive phenotype
for a trait is heterozygous or
homozygous for the that trait.
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Reading Essentials • Introduction to Genetics and Patterns of Inheritance
Punnett Squares
In the early 1900s, Dr. Reginald Punnett developed a square to
predict possible offspring of a cross between two known
genotypes. Punnett squares are useful for keeping track of
genotypes in a cross.
What information does a Punnett square contain?
A Punnett square can help you predict the genotype and
phenotype of the offspring. The genotype of one parent is written
vertically, on the left side of the Punnett square. The genotype of
the other parent is written horizontally, across the top. A Punnett
square for a monohybrid cross contains four small squares. Each
small square represents a possible combination of alleles in the
The Punnett square below shows the results of Mendel’s
experiment with seed color. The Punnett square shows that four
different genotypes are possible—one YY, two Yy, and one yy.
The genotypic ratio is 1:2:1.
How is a Punnett square used for two traits?
Punnett squares also can be used to predict the results of a
dihybrid cross. A Punnett square for a dihybrid cross is larger.
It has 16 boxes to represent 16 allele combinations.
Genetics follows the rules of probability, or chance. It is like
flipping a coin. The probability of flipping heads is one out of
two. Because of chance, if you flip a coin 100 times, it might not
land heads exactly 50 times, but it will be close.
It is the same in genetics. A cross might not give a perfect 3:1 or
9:3:3:1 ratio. The larger the number of offspring, the more closely
the results will match the ratio predicted by the Punnett square.
9. Define Circle the
genotypes in the small
squares that will give a
yellow-seed phenotype.
What will be the phenotypic
ratio in the offspring?
8. Identify What is one
purpose of a Punnett
Copy female alleles across
each row. Copy male alleles
down each column. Always
list the dominant trait first.
Place the
male alleles
along the top.
Place the
female alleles
along the side.
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Reading Essentials • Introduction to Genetics and Patterns of Inheritance
2 Genetic Recombination and Gene
Genetics is like a game of cards. In meiosis, chromosomes are
shuffled and sorted. On the lines below, explain the chances of a
player getting the same cards two games in a row. In this lesson,
you will learn about the independent assortment of chromosomes
that occurs during meiosis.
Genetic Recombination
During meiosis, genes are combined in new ways. Genetic
recombination occurs when crossing over and independent
assortment produce new combinations of genes.
Recall that independent assortment occurs in meiosis when
chromosomes separate randomly. The number of possible gene
combinations due to independent assortment can be calculated
using the formula 2n
, where n equals the number of chromosome
Pea plants have 7 pairs of chromosomes. The possible
combinations of these chromosomes would be 27, or 128.
Fertilization further increases the number of combinations.
During fertilization, any possible male gamete can fertilize any
possible female gamete. The number of combinations after
fertilization would be 2n × 2n
. For peas, this number is 16,384, or
128 × 128.
In people, the possible combinations of chromosomes are
223 × 223—over 70 trillion. Crossing over increases genetic
recombination even more.
Highlight the main ideas under
each heading. State each main
point in your own words.
1. Calculate The fruit fly has
four chromosome pairs.
How many possible
combinations of
chromosomes can be
produced by meiosis and
• how meiosis produces
genetic recombination
• how gene linkage is used to
make chromosome maps
• why polyploidy is important
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Reading Essentials • Introduction to Genetics and Patterns of Inheritance
Gene Linkage
Chromosomes contain many genes. Genes that are located
close together on the same chromosome are said to be linked.
This means they usually travel together during gamete formation.
Linked genes do not segregate independently. They are an
exception to Mendel’s law of independent assortment.
Occasionally, linked genes separate due to crossing over.
Crossing over occurs more frequently between genes that are far
apart than between genes that are close together.
What does a chromosome map show?
The relationship between crossing over and chromosome
distance is very useful. The distance between two genes can be
estimated by the frequency of crossing over that occurs between
them. Scientists use cross-over data to create a drawing of genes
along a chromosome. The drawing, called a chromosome map,
shows the order of genes on a chromosome. The first
chromosome maps were published in 1913 for fruit-fly crosses.
One is shown in the figure below.
Most organisms have diploid cells—cells with two chromosomes
in each cell. Some species have polyploid cells. Polyploidy (PA lih
ploy dee) means the cells have one or more extra sets of all
chromosomes. For instance, a triploid organism has three
complete sets of chromosomes in each cell. It is designated 3n.
Polyploidy occurs in only a few animals, such as earthworms
and goldfish. It is always lethal in humans. Polyploidy is common
in flowering plants. Polyploid plants are often bigger and more
vigorous. Many food plants, such as wheat (6n), oats (6n), and
sugarcane (8n), are polyploid.
4. Identify Which two genes
are not likely to cross over?
(Circle your answer.)
a. yellow body color and
vermilion eye color
b. white eye color and
vermilion eye color
5. Identify Name two
organisms that have
3. Explain What event causes
linked genes to separate?
Get It?
2. Analyze the effect of
crossing over on linked
Get It?
6. Explain why plant growers
often select for polyploid
y w v m r
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Reading Essentials • Introduction to Genetics and Patterns of Inheritance
• how inbreeding differs from
• how to use test crosses and
a Punnett square to find the
genotypes of organisms
After you read this lesson,
create a five-question quiz
from what you have learned.
Then, exchange quizzes with
another student. After taking
the quizzes, review your
answers together.
1. Name an advantage
of hybridization.
3 Applied Genetics
Imagine that you could design the perfect dog. What color would
it be? Would it be big or small? On the lines below, describe the
traits your dog would have. In this lesson, you will learn how
selective breeding produces certain traits.
Selective Breeding
For thousands of years, people have been breeding animals
and plants to have certain traits. For instance, some dogs, such
as huskies, have been bred to be strong runners. Other dogs,
such as Saint Bernards, have been bred to have a good sense of
People have also bred plants, such as tomatoes, apples, and
roses, to taste better, resist disease, or produce fragrant flowers.
Selective breeding is the process used to breed animals and
plants to have desired traits. As a result of selective breeding,
desired traits become more common.
What is hybridization?
A hybrid is an organism whose parents each have different
forms of a trait. For instance, a disease-resistant tomato plant
can be crossed with a fast-growing tomato plant. The offspring
of the cross would be a tomato plant that has both traits.
The hybrid is disease resistant and grows quickly.
Hybridization is the process of making a hybrid organism.
Hybridization is expensive and takes a long time, but it is a good
way to breed animals and plants with the right combination
of traits.
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Reading Essentials • Introduction to Genetics and Patterns of Inheritance
When are test crosses performed?
An orchard owner might use a test cross to find out the
genotype of a white-grapefruit tree. In grapefruits, white
color is a dominant trait and red color is a recessive trait.
A red-grapefruit tree has two recessive genes (ww). A whitegrapefruit tree might have two dominant genes (WW), or it might
have one dominant gene and one recessive gene (Ww).
4. Label Fill in the phenotype
with the word white or red
for each genotype.
Genotype Phenotype
Homozygous dominant (WW)
Homozygous recessive (ww)
Heterozygous (Ww)
Get It?
2. Describe the disadvantages
associated with hybridization
and inbreeding.
3. Explain What is the
purpose of a test cross?
How is inbreeding used?
Inbreeding is another example of selective breeding.
Inbreeding occurs when two closely related organisms that
both display the desired trait are bred. Inbreeding can be used
to ensure that the desired trait is passed on. Inbreeding can also
eliminate traits that are not desired.
Purebred animals are created by inbreeding. Clydesdale horses
are an example of a purebred animal. Clydesdale horses were first
bred in Scotland hundreds of years ago. They were bred for use
as farm horses that could pull heavy loads. All Clydesdales have
the traits of strength, agility, and obedience.
A disadvantage of inbreeding is that harmful traits can
be passed on. Harmful traits are usually carried on recessive
genes. Both parents must pass on the recessive genes for the
harmful traits to appear in the offspring. Inbreeding increases
the chance that both parents carry the harmful traits.
Test Cross
Breeders need a way to determine the genotype of the organisms
they want to cross before creating a hybrid. They use test crosses to
find out the genotype of an organism. In a test cross, an organism
whose genotype for a desired trait is unknown is crossed with an
organism that has two recessive genes for the trait.
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Reading Essentials • Introduction to Genetics and Patterns of Inheritance
How does a test cross reveal the genotype?
The orchard owner decides to do a test cross to find out the
genotype of a white grapefruit tree. The white grapefruit tree is
crossed with a red grapefruit tree. The orchard owner uses a
Punnett square to understand the results of the cross.
The figure below shows a Punnett square for the test cross
if the white grapefruit tree is homozygous, meaning it has two
dominant genes (WW) for white fruit. All the offspring from
the test cross will be heterozygous, meaning they will have one
dominant and one recessive gene (Ww). All the offspring of the
test cross are white grapefruit trees.
5. Evaluate If you planted 100
seeds from this test cross,
about how many would
be white? How many would
be red?
What if the test cross involved a heterozygous tree?
The figure below shows a Punnett square for the test cross
if the white grapefruit tree is heterozygous (Ww). Half the
offspring from the test cross will be white (Ww). Half the
offspring from the test cross will be red (ww). TAKE A LOOK
6. Calculate If you planted
100 seeds from this test
cross, about what
percentage would be white?
What percentage would be
w Ww Ww
w Ww Ww
white grapefruit
red grapefruit
w Ww ww
w Ww ww
white grapefruit
red grapefruit
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Reading Essentials • Introduction to Genetics and Patterns of Inheritance
4 Basic Patterns of Human Inheritance
A family tree shows how people in a family are related. On the
lines below, list people who might appear in a family tree. Then
read the lesson to learn how scientists trace inheritance through
several generations of a family.
Review the table below and recall that a recessive trait is
expressed when the person is homozygous recessive for that trait.
A person with at least one dominant allele will not express the
recessive trait. A person who is heterozygous for a recessive
disorder is called a carrier.
• how to determine if an
inherited trait is dominant or
• examples of dominant and
recessive disorders
After you read this lesson,
create a quiz based on what
you have learned. Then be
sure to answer the quiz
1. Identify Circle the term that
describes the genotype of a
person who expresses a
recessive trait.
Term Description
Homozygous An organism with two of the same alleles for
a particular trait is said to be homozygous for
that trait.
Heterozygous An organism with two different alleles for a
particular trait is said to be heterozygous for that
trait. When alleles are present in the heterozygous
state, the dominant trait will be observed.
Scientists use a diagram called a pedigree to trace inheritance
of a trait through several generations. A pedigree uses symbols
to illustrate inheritance of the trait.
A sample pedigree is shown in the figure on the next page. In
the top row, the two symbols connected by a horizontal line are
the parents. Their children are listed below them, oldest to
youngest from left to right.
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Reading Essentials • Introduction to Genetics and Patterns of Inheritance
Roman numerals are used to represent generations—I for the
parents, II for the children, and so on. Arabic numbers are used
to represent the individuals within a generation.
Normal female
Key to Symbols
Normal male
Female who expresses
the trait being studied
Female who is a carrier
for the particular trait
1 2 3 4
2 Male who expresses
the trait being studied
Male who is a carrier
for the particular trait
Roman numerals — Generations
Arabic numerals — Individuals in
a certain generation
Example Pedigree
2. Calculate What percentage
of the children in this family
inherited Tay-Sachs
Get It?
3. Explain how symbols are
used to represent
individuals in a pedigree.
Analyzing Pedigrees
The figure below is a pedigree showing the inheritance
of Tay-Sachs disease, a recessive disorder. The pedigree shows
that two parents who do not have Tay-Sachs disease can have
a child who has the disorder.
How is the inheritance of a dominant disorder shown
on a pedigree?
The pedigree below shows the inheritance of the dominant
disorder, polydactyly (pah lee DAK tuh lee)—extra fingers
and toes. A person with dominant disorders could be
homozygous or heterozygous for the trait. A person who does
not have polydactyly would be homozygous recessive for
the trait.
4. Identify Do any of the
grandchildren in this family
have polydactyly?
1 2 3 4
Carriers for Tay-Sachs
1 2
1 2
1 3 4 5 6 7
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Reading Essentials • Introduction to Genetics and Patterns of Inheritance
How are genotypes deduced?
A pedigree can be used to learn the genotype of a person.
The genotype is determined by observing the phenotypes,
or physical traits, of a person.
Genetic counselors use pedigrees to determine if an inherited
trait is dominant or recessive. Dominant traits are easy to
recognize. Recessive traits are more difficult because people
who carry the allele do not always show the trait.
Can genetic disorders be predicted?
Scientists can use pedigrees to predict whether a person
in a family will get a genetic disorder. Scientists have to follow
several people for many generations to accurately study
a disorder. Good record keeping within a family can help
scientists predict the inheritance of a disorder.
Types of Recessive Genetic Disorders
What causes albinism?
Albinism is a recessive disorder found in people and animals.
In humans, it is caused by the absence of the skin pigment
melanin in hair and eyes. People with albinism have white hair,
pale skin, and pink eyes. They need to protect their skin from the
Sun’s ultraviolet rays.
What causes galactosemia?
Galactosemia (guh lak tuh SEE mee uh) is a recessive genetic
disorder. It causes intolerance of the sugar galactose. Milk
contains the sugar lactose. During digestion, lactose breaks down
into galactose and glucose, the sugar used by the body for energy.
People with galactosemia lack the enzyme needed to break down
What is cystic fibrosis?
Cystic fibrosis is a recessive genetic trait. Most chloride ions are
not absorbed into cells but are excreted in sweat. Without the
chloride ions in cells, very little water diffuses from cells. This
causes the mucus secreted by many areas of the body to be
abnormally thick. The mucus interferes with digestion, clogs
ducts in the pancreas, and blocks air pathways in the lungs.
Patients with cystic fibrosis often get infections because of excess
mucus in their lungs. Treatment includes physical therapy,
medicine, special diets, and replacement digestive enzymes.
Genetic tests can determine if the recessive gene is present.
6. Explain Why are recessive
traits difficult to study?
Get It?
5. Analyze What can be
determined about the
genotypes of the parents of
an individual who expresses
a recessive trait?
Make a vocabulary book
and label each tab with the
name of a different genetic
disorder. Use it to organize
your notes on genetic
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Reading Essentials • Introduction to Genetics and Patterns of Inheritance
9. Explain How can scientists
determine if achondroplasia
developed from a new
Get It?
8. Compare the chances of
inheriting a dominant
disorder to the chances
of inheriting a recessive
disorder if you have one
parent with the disease.
What is Tay-Sachs disease?
Tay-Sachs (TAY saks) disease is a recessive genetic disorder.
Tay-Sachs disease (TSD) is more common among Jews whose
ancestors are from eastern Europe.
People with TSD are missing an enzyme needed to break down
fatty acids called gangliosides. Normally, gangliosides are made
and then destroyed as the brain develops. In people with TSD,
gangliosides build up in the brain, causing mental deterioration.
Children born with TSD usually die by age five. Currently, there
is no cure.
Dominant Genetic Disorders
Not all genetic disorders are recessive. Some are caused by
dominant alleles. People who do not have the disorder are always
homozygous recessive, meaning they carry two recessive genes
for the trait.
What happens in Huntington’s disease?
Huntington’s disease is a dominant genetic disorder that affects
the nervous system. It is rare. Symptoms occur when the person
is between 30 and 50 years old. Symptoms are gradual loss of
brain function, uncontrollable movements, and emotional
disturbances. Genetic tests can tell people whether they have the
gene for Huntington’s disease, but there is currently no treatment
or cure.
What is achondroplasia?
Achondroplasia (a kahn droh PLAY zhee uh) is a dominant
genetic disorder that is also known as dwarfism. People with this
disorder have a small body size and short limbs. They grow to an
adult height of about 1.2 m (4 ft).
About 75 percent of people with achondroplasia have parents
of average size. Because the gene is dominant, parents who are
average size do not have the gene. Therefore, when average-sized
parents have a child with achondroplasia, the condition occurs
because of a new mutation.
7. Explain Why do
gangliosides build up
in the brain of people with
Tay-Sachs disease?
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Reading Essentials • Introduction to Genetics and Patterns of Inheritance
5 Complex Patterns of Inheritance
Cats can look different from one another because of differences in
their coats. On the lines below, describe differences you have seen
in the coats of cats. Then read the lesson to learn more about
complex inheritance patterns.
Incomplete Dominance
Not all traits follow Mendel’s rules. Some traits are not
dominant or recessive. Sometimes, the heterozygous organism
has a mixed phenotype. Incomplete dominance occurs when
the heterozygous phenotype is an intermediate phenotype
between the two homozygous phenotypes.
An example of incomplete dominance occurs in snapdragon
flowers. Red-flowered snapdragons (CR CR) can be crossed with
white-flowered snapdragons (CW CW) to produce offspring
with pink flowers (CR CW). When heterozygous F1 generation
snapdragon plants (CR CW) self-fertilize, the offspring have a 1:2:1
ratio of red, pink, and white flowers.
In Mendel’s experiments with pea plants, heterozygous pea
plants expressed only the dominant allele. Codominance occurs
when a heterozygous organism expresses both alleles. Sickle-cell
anemia is an example of codominance. People who are
heterozygous for the sickle-cell trait have both normal and sickleshaped cells.
• the difference between sexlinked and sex-limited
• how environment can
influence a trait
Highlight each question head.
Then highlight the answer
to the question.
1. Define What is
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Reading Essentials • Introduction to Genetics and Patterns of Inheritance
What happens in sickle-cell disease?
Sickle-cell disease is common in people of African descent.
Sickle-cell disease affects red blood cells and their ability to
transport oxygen. Changes in the protein in red blood cells cause
those red blood cells to change from a normal disc shape to a
sickle or C shape.
Sickle-cell disease is a codominant trait. People who are
heterozygous for the trait make both normal and sickle-shaped
cells. The normal cells compensate for the sickle-shaped cells.
How does sickle-cell disease relate to malaria?
Sickle-cell disease is found in areas of Africa where malaria
occurs. Scientists have discovered that people who are
heterozygous for the sickle-cell trait are resistant to malaria.
Because the sickle-cell gene helps people resist malaria, they are
more likely to pass the sickle-cell trait on to their offspring.
Multiple Alleles
So far you have learned about traits that result from a gene with
two alleles. Some traits are controlled by a gene that has multiple
alleles. Blood groups in humans is an example of a multiple
allele trait.
How are blood types produced?
There are four blood types in people: A, AB, B, or O.
The four types result from the interaction of three different
alleles, as shown below. The allele I
A produces blood type A.
B produces blood type B. The allele i is recessive and produces
blood type O. Type O is the absence of AB alleles. People with
one I
A and one I
allele have blood type AB. Blood types are
examples of multiple alleles and codominance.
Rh factors are also in blood. One factor is inherited from each
parent. Rh factors are either positive or negative (Rh+ or Rh−); the
Rh+ is dominant.
2. Describe What effect does
sickle-cell disease have on
red blood cells?
4. Evaluate What phenotype
results from a genotype
of I
Get It?
3. Explain how the genetic
traits carried on multiple
alleles can lead to a wide
range of characteristics in
Genotypes Resulting Phenotypes
Type A
Type A
Type B
Type B
Type AB
Type O
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Reading Essentials • Introduction to Genetics and Patterns of Inheritance
What genes control coat color in rabbits?
The fur color of rabbits is another trait controlled by multiple
alleles. In rabbits, four alleles control coat color: C, c
ch, c
, and c.
The alleles are dominant in varying degrees. The hierarchy can
be written as C > c
> c
> c.
Allele C is dominant to all other alleles and results in a dark
gray coat color. Allele c
ch is dominant to c
, and c
is dominant
to c. Allele c is recessive and results in an albino when the
genotype is homozygous recessive.
Multiple alleles increase the possible number of genotypes and
phenotypes. Two alleles have three possible genotypes and two
possible phenotypes. Four alleles have ten possible genotypes and
can have five or more phenotypes.
Epistasis (ih PIHS tuh sus) occurs when one allele hides the
effects of another allele. Coat color in Labrador retrievers is a trait
controlled by epistasis. Labrador coats vary from yellow to black.
Two different genes control coat color. The dominant allele E
determines whether the coat will have dark pigment. A dog with
genotype ee will not have any pigment. The dominant allele B
determines how dark the pigment will be. If the genotype is EEbb
or Eebb the coat will be chocolate. If the genotype is eebb, eeBb,
or eeBB the coat will be yellow because the e allele hides the
effects of the dominant B allele.
Dosage Compensation
There are two types of sex chromosomes—X and Y. A person’s
gender is determined by the sex chromosomes present in the egg
and sperm cell. Females inherit two X chromosomes. Males
inherit one X and one Y chromosome.
In humans, the X chromosome carries genes needed by males
and females. The Y chromosome mainly carries genes needed
to develop male characteristics. Does this mean that females get a
double dose of the X chromosome? No, because in females, one
of the X chromosomes in every body cell stops working. This is
called dosage compensation or X-inactivation.
5. Evaluate What allele is
dominant over cch?
a. ch
b. c
c. C
6. Identify A person has 22
pairs of autosomes and two
X chromosomes. What is
the person’s gender?
Copyright © McGraw-Hill Education
Reading Essentials • Introduction to Genetics and Patterns of Inheritance
How is coat color determined in calico cats?
The coat color of calico cats is controlled by the random
inactivation of X chromosomes. Orange patches are formed
when an X chromosome carrying the allele for black coat color
is turned off. Black patches are formed when an X chromosome
carrying the allele for orange coat color is turned off.
What are Barr bodies?
Canadian scientist Murray Barr first observed inactivated
X chromosomes, now known as Barr bodies. Barr bodies appear
as dark objects in the cell nuclei of female mammals.
Sex-Linked Traits
Traits controlled by genes on the X chromosome are called
sex-linked traits or X-linked traits. Males have only one
X chromosome, so they are affected more than females by
recessive sex-linked traits. Females are less likely to express
a recessive sex-linked trait because one X chromosome may mask
the effect of the recessive trait on the other X chromosome.
How is red-green color blindness inherited?
The trait for red-green color blindness is a recessive
sex-linked trait. People who are color blind cannot see the colors
red and green. About 8 percent of males in the United States are
red-green color blind. Examine the Punnett square below to see
how red-green color blindness is inherited.
How is hemophilia inherited?
Normally, when a person is cut, the bleeding stops quickly.
Hemophilia is a recessive sex-linked disorder that slows blood
clotting. Hemophilia is more common in males. Until the
discovery of clotting factors in the twentieth century, most men
with hemophilia died at an early age. Safe methods of treating the
disorder now allow for a normal life span.
8. Draw Conclusions Why is
a recessive sex-linked trait
less likely to occur in
females than in males?
9. Predict Circle the genotype
that represents a color-blind
Get It?
7. Summarize dosage
compensation and its
XB Normal
Red-green color blind
Y chromosome
XB Normal
Red-green color blind
Y chromosome
Copyright © McGraw-Hill Education
Reading Essentials • Introduction to Genetics and Patterns of Inheritance
Polygenic Traits
So far you have learned about traits that are controlled by one
gene with different alleles. Polygenic traits develop from the
interaction of multiple pairs of genes. Many traits in humans are
polygenic, including skin color, height, eye color, and fingerprint
Environmental Influences
The environment influences many traits. Factors such
as sunlight, temperature, and water can affect an organism’s
phenotype. For example, the gene that codes for the production
of color pigment in Siamese cats functions only under cooler
conditions. Cooler parts of the cat’s body, such as the ears, nose,
feet, and tail, are darker. The warmer parts of the body, where
pigment production is inhibited, are lighter.
Environmental factors also include an organism’s actions.
Heart disease can be inherited, but diet and exercise also strongly
influence the disease. An organism’s actions are considered part
of the environment because they do not come from genes.
Twin Studies
Scientists can learn about inheritance patterns by studying
twins. Twin studies often reveal how genes and the environment
affect phenotype.
Identical twins have identical genes. If a trait is inherited, both
identical twins will have the trait. Scientists presume that traits
that are different in identical twins are strongly influenced by the
environment. The percentage of identical twins who both have
the same trait is called a concordance rate, as shown in the graph
below. The higher the concordance rate, the stronger the genetic
10. List an example of a
polygenic trait.
11. Evaluate Circle the trait
that shows the strongest
genetic influence.
in females
in males
Depression Reading
Concordance Rates
Identical twins Fraternal twins

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