Download Chapter 5: Extensions of Mendelian Inheritance

Chapter 5: Extensions of Mendelian Inheritance
Student Learning Objectives
Upon completion of this chapter you should be able to:
1. Understand the relationship between gene expression and the dominant/recessive
phenotype.
2. Understand the different patterns of Mendelian inheritance involving single genes and
how to solve relevant problems.
3. Recognize how traits can be influenced by the environment.
4. Differentiate between sex-linked, sex-limited, and sex-influenced patterns of inheritance.
5. Understand how lethal alleles may result in inheritance patterns with unexpected results.
6. Recognize how gene interactions, such as epistasis, can alter the predicted 9:3:3:1 ratio of
a dihybrid cross.
5.1 Overview of Simple Inheritance Patterns
Overview
The first section of this chapter introduces the various patterns of inheritance that involve
single genes. These will be discussed in detail in the remaining sections. At this point, study
Table 5.1 closely to become familiar with all these patterns. It is important to note that pure
Mendelian inheritance is rare. Instead, patterns of inheritance are clouded by many factors,
including the environment and protein function. One of the most important things to consider as
you progress through this chapter is the relationship between the observed inheritance pattern and
its molecular basis. By doing so, you will quickly come to realize that the behavior of the proteins
encoded by genes is the major factor in determining the phenotype of the individual.
Outline of Key Terms
Mendelian inheritance (Simple Mendelian inheritance)
Focal Points

Types of Mendelian inheritance patterns involving single genes (Table 5.1)
Exercises and Problems
Name the pattern of inheritance based on its description:
_______
_______
_______
_______
_______
1.
2.
3.
4.
5.
The heterozygote’s phenotype is intermediate between those of the homozygotes.
The heterozygote has a trait that is more beneficial than either homozygote.
The heterozygote expresses both alleles simultaneously.
The allele has the potential to cause the death of an organism.
The trait occurs in only one of the two sexes.
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5.2 Dominant and Recessive Alleles
Overview
The second section examines two main ideas: 1) what makes an allele dominant or
recessive; and 2) how dominant alleles may not always exert their effects.
The section opens with a discussion of wild-type and mutant alleles. In some instances,
more than one wild-type allele can occur (Refer to Figure 5.1). This phenomenon is termed
genetic polymorphism (Figure 5.1). In general, recessive alleles are due to mutations that result in
a reduction or loss-of-function of the encoded protein. Dominant alleles, on the other hand, are
most commonly caused by gain-of-function mutations, dominant negative mutations, or
haploinsufficiency. The last part of this section addresses two phenomena that can influence
traits: 1) incomplete penetrance, where an allele that is expected to be expressed is not expressed
(Figure 5.3); and 2) expressivity, which refers to the degree to which a trait is expressed.
Outline of Key Terms
Dominant mutant alleles
Gain-of-function mutations
Dominant negative mutations
Haploinsufficiency
Incomplete penetrance
Expressivity
Wild-type alleles
Genetic polymorphism
Mutant alleles
Focal Points


A comparison of protein levels among various genotypes of flowers (Figure 5.2)
Polydactyly, a dominant trait that shows incomplete penetrance (Figure 5.3)
Exercises and Problems
For questions 1 to 5, match each of the following to its correct definition.
_____ 1.
Wild-type allele
_____ 2.
Incomplete penetrance
_____ 3.
Dominant negative mutation
_____ 4.
Mutant allele
_____ 5.
Haploinsufficiency
a. These are altered alleles that tend to be rare in natural populations.
b. A heterozygote (with one functional and one inactive allele) exhibits an abnormal phenotype.
c. The protein encoded by the mutant gene acts antagonistically to the normal protein.
d. The dominant phenotype is not expressed even if a dominant allele is present.
e. The most prevalent form of an allele in a population.
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5.3 Environmental Effects on Gene Expression
Overview
It is important to note that genes are not the only determinant of an organism’s
phenotype. This section discusses three examples: 1) coat color in the arctic fox, which is white in
the winter and brown in the summer (Figure 5.4A); 2) the disease phenylketonuria in humans,
which can be avoided if individuals are diagnosed early and follow a restricted diet free of
phenylalanine (Figure 5.4B); and 3) facet number in the eyes of fruit flies, which can change
based on the temperature, even in genetically-identical individuals (Figure 5.4C).
Outline of Key Terms
Norm of reaction
Focal Points

Variation in the expression of traits due to environmental effects (Figure 5.4)
Exercises and Problems
Complete the following sentences with the most appropriate word or phrase:
1.
2.
3.
4.
Coat color in the arctic fox is caused by a ______-______ allele.
Phenylketonuria (PKU) is caused by a defect in the gene that encodes the enzyme _______.
Individuals with PKU are placed on restricted diets that are free of _______.
The term _______ refers to the effects of environmental variation on a phenotype.
5.4 Incomplete Dominance, Overdominance, and Codominance
Overview
This section takes a closer look at three types of inheritance patterns in which the
heterozygote shows a phenotype that is different from those of the two homozygotes. The first of
these types is incomplete dominance, where the heterozygote exhibits an intermediate phenotype.
Indeed, it is important at this point to note the description of a trait as dominant or incompletely
dominant actually depends on the level at which phenotype is examined (Refer to Figure 5.6).
The second type is overdominance, which occurs when heterozygotes have superior traits
to those of the corresponding homozygotes. Three possible explanations for this are given in
Figure 5.8. The third inheritance pattern discussed in this section is codominane. It refers to the
phenomenon in which a heterozygous individual expresses both alleles. The text first addresses
the concept of multiple alleles, using the ABO blood type. These blood types are determined by
antigens on the surface of red blood cells (Refer to Figure 5.9). The synthesis of these antigens is
controlled by three alleles (IA, IB, and i), which exhibit various levels of dominance: IA and IB are
codominant, and i is recessive to both of them.
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Outline of Key Terms
Incomplete dominance
Overdominance
Heterozygote advantage
Homodimer
Codominance
Multiple alleles
Focal Points



Incomplete dominance in the four-o’clock plant (Figure 5.5)
Inheritance of sickle cell disease (Figure 5.7)
ABO blood type (Figure 5.9)
Exercises and Problems
1. Is incomplete dominance an example of blending? Explain your answer.
2. If a red and pink four o’clock plant are crossed, what will be the phenotypic and genotypic
ratio of the F1 generation?
For questions 3 to 5, complete the following sentences with the most appropriate word or phrase:
3. In ABO blood groups, type _____ is the universal donor and type ____ the universal recipient.
4. The IA and IB alleles encode enzymes called _________.
5. Overdominance is also also called ________.
For questions 6 to 8, select the molecular basis of each of the following patterns of inheritance.
_____ 6.
Codominance
_____ 7.
Incomplete dominance
_____ 8.
Overdominance
a. Heterozygotes produce proteins that function over a wider range of conditions.
b. The proteins produced different alleles function in slightly different ways resulting in both
being expressed in the heterozygote.
c. The amount of protein produced by a single dominant allele is not sufficient to produce the
normal phenotype.
For questions 9 to 11, use the following information: Jack has blood type A, and his father has
blood type O. Jill, Jack’s wife, has blood type AB?
9. What is the probability that the couple’s first child has blood type A?
10. What is the probability that the couple’s first child is a son with blood type AB?
11. What is the probability that the couple’s first two children have blood type B?
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5.5 Sex-Influenced and Sex-Limited Inheritance
Overview
In Chapter 4, we discussed sex-linked inheritance in humans, which is based on genes
located on the sex chromosomes. It follows three basic models: X-linked, Y-linked, and
pseudoautosomal. This chapter addresses sex-influenced and sex-limited inheritance, both of
which involve genes found on autosomes. In sex-influenced traits, heterozygous males and
females have different phenotypes. An example is pattern baldness in humans. Sex-limited traits,
on the other hand, are expressed in only one sex. An example is the feathering pattern in chicken.
Outline of Key Terms
Sex-influenced inheritance
Sex-limited inheritance
Sexual dimorphism
Focal Points


Sex-influenced trait: Pattern baldness in humans (Figures 5.10 and 5.11)
Sex-limited trait: Feathering pattern in chicken (Figure 5.12)
Exercises and Problems
Complete the following sentences with the most appropriate word or phrase:
The gene that affects pattern baldness encodes an enzyme called (1) _______, which
converts the hormone testosterone to (2) _______. The allele that causes pattern baldness results
in overexpression of this enzyme. A rare tumor of the (3) _______ can cause the secretion of
abnormally large amounts of testosterone. If this occurs in a woman who is (4) _______, she will
become bald.
Questions 5 and 6 refer to a bald woman and a nonbald man who are planning a family.
5. What is the probability that the couple’s first child is a son that will become bald as an adult?
6. What is the probability that the couple’s first child is a daughter that will be bald as an adult?
5.6 Lethal Alleles
Overview
This section addresses the concept of lethal alleles, which can potentially lead to the
death of an organism. These alleles come in different forms, including temperature-sensitive,
conditional, and semilethal. Lethal alleles may result in inheritance patterns that yield unexpected
ratios (Refer to Figure 5.13).
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Outline of Key Terms
Lethal alleles
Essential gene
Non-essential gene
Age of onset
Conditional lethal alleles
Temperature-sensitive (ts) lethal alleles
Semilethal alleles
Focal Points

The Manx cat, which carries a lethal allele (Figure 5.13)
Exercises and Problems
For questions 1 to 5, match each of the following to its correct definition.
_____ 1.
Conditional lethal allele
_____ 2.
Nonessential gene
_____ 3.
Essential gene
_____ 4.
Semilethal allele
_____ 5.
Lethal allele
a. An allele that kills an organism only in certain environments.
b. An allele that causes the death of an organism.
c. A lethal allele that only acts in some individuals.
d. A gene that encodes a protein that when absent causes the death of the organism.
e. A gene which is not absolutely required for survival.
5.7 Pleiotropy
Overview
This short section deals with pleiotropy, the phenomenon where a single gene can have
multiple effects on the phenotype of the organism. Pleiotropy can occur for several reasons,
including: 1) A gene may affect cell function in more than one way; 2) A gene may be expressed
in different cell types in a multicellular organism; and 3) A gene may be expressed at different
stages of development.
Outline of Key Terms
Pleiotropy
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Exercises and Problems
Complete the following sentences with the most appropriate word or phrase:
An example of a pleiotropic mutation involves the disease (1) ________. The gene in
question encodes a protein called CFTR, which transports (2) ______ ions across epithelial cell
membranes. The mutation that causes this genetic disease affects several parts of the body. The
most severe symptoms is thick (3) _______ in the lungs, that occurs because of a water
imbalance. In addition, affected males may be infertile, because the (4) ______, the tubules that
transport sperm from the testes, may be absent or undeveloped.
5.8 Gene Interactions
The previous sections examined deviations from Mendelian inheritance based upon
single genes. You should now also recognize that many of these patterns of inheritance were due
to the proteins that were being produced by the genes. However, many traits are under the
influence of more than one gene. In many cases, several genes produce enzymes that are involved
in metabolic pathways. If a mutation in a gene early in the pathway produces a loss of function,
then this may mask the phenotype of genes that contribute alleles later in the pathway (for
example, see the left column on page 105). This is called epistasis.
As you progress through this section, note how the gene interactions being described alter
the 9:3:3:1 ratio of a Mendelian two-factor (dihybrid) cross. Before proceeding, carefully
examine the table below of a two-factor cross involving two heterozygous individuals.
Gene A
A–
aa
A–
aa
Gene B
B–
B–
bb
bb
Phenotypic Ratio
9/16
3/16
3/16
1/16
Notice that the 9/16th aspect of ratio occurs if a dominant allele is inherited for both traits, while a
3/16th ratio occurs if a dominant allele is inherited for only one of the traits. 1/16th of the time the
phenotype is recessive. By recognizing these numbers it is usually easy to identify epistatic
interactions, since the masking of one of the traits will cause the ratios to be combined. For
example, a 9:7 ratio indicates that the A–bb, aaB–, and aabb ratios have been combined together.
This will be useful later in additional discussions of gene interactions.
Another example of a gene interaction is complementation (Refer to Figure 5.14). This
refers to the production of offspring with a wild-type phenotype from parents that both display
the same or recessive phenotypes. This section concludes with a discussion of gene redundancy,
which is the phenomenon where a gene compensates for lack of function of another. The text
provides a molecular explanation (Figure 5.15) and an example (Figure 5.16).
Outline of Key Terms
Gene interaction
Epistasis
Recessive epistasis
Complementation
Gene redundancy
Paralogs
Gene knockout
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Focal Points



Types of Mendelian inheritance patterns involving two genes (Table 5.3)
A cross between two different white varieties of the sweet pea (Figure 5.14)
The phenomenon of gene redundancy (Figures 5.15 and 5.16)
Exercises and Problems
For questions 1 to 4, complete the following sentences with the most appropriate word or phrase:
1. ________ occurs when the alleles of one gene mask the phenotypic effects of another gene.
2. ________ occurs when two different parents that express the same phenotype produce
offspring with a wild-type phenotype.
3. ________ occurs when one gene compensates for the loss of function of another gene.
4. ________ are duplicated genes which are not identical due to the accumulation of mutations.
5. In crosses involving the sweet pea, two white flowered plants are crossed. Indicate whether the
following offspring are possible.
a. all white offspring
b. all purple offspring
c. a combination of purple and white offspring
For questions 6 and 7, use the following information: In the shepherd’s purse plant, capsule shape
is determined by two gene loci, T and V. Triangular shape requires at least one dominant allele.
Plants that are ttvv homozygotes have ovate capsules.
6. A plant with genotype TtVv is self-fertilized. What is the phenotypic ratio in the offspring?
7. A plant with genotype TtVv is test-crossed. What is the phenotypic ratio in the offspring?
Chapter Quiz
1. The Manx phenotype in cats is caused by a dominant allele that is lethal in the homozygous
state. If two Manx cats are crossed, what phenotypic ratio is expected in kittens that are born?
a. all Manx
b. 3 Manx : 1 normal
c. 2 Manx : 1 normal
d. 1 Manx : 1 normal
e. all normal
2. Expressivity is most often associated with which of the following?
a. multiple allele systems
b. heterozygote advantage
c. incomplete penetrance
d. incomplete dominance
e. none of the above
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3. The ABO blood groups in humans are an example of ________.
a. multiple allele systems
b. epistatic interactions
c. gene dosage
d. simple Mendelian inheritance
e. incomplete dominance
4. The most common allele in the population is called the _______.
a. mutant allele
b. essential allele
c. dominant allele
d. wild-type allele
e. recessive allele
5. A trait that produces a 9:7 ratio in the F2 generation of a two-factor cross is most likely
exhibiting which of the following?
a. gene redundancy
b. epistatic interactions
c. overdominance
d. multiple allele systems
e. sex-limited inheritance
6. An autosomal gene in humans has four alleles. How many different genotypes are possible?
a. 4
b. 8
c. 10
d. 12
e. 16
7. A heterozygote that has a selective advantage over the homozygous dominant individual is an
example of ________.
a. gene interaction
b. codominance
c. temperature sensitive lethals
d. overdominance
8. Which of the following CANNOT possibly be the offspring of a man with type A blood and a
woman with type B blood?
a. a child with type A
b. a child with type B
c. a child with type AB
d. a child with type O
e. choose this answer if all of the above are possible
9. A pattern-bald woman is married to a man with normal hair. Which of the following statements
about their adult offspring is TRUE?
a. All of their children are expected to be nonbald
b. Half of their children of either sex are expected to be bald
c. Half of their sons are expected to be bald, but none of their daughters
d. All of their sons are expected to be bald; but none of their daughters
e. All of their daughters are expected to be bald; but none of their sons
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10. Phenotypic blending is the result of ________.
a. incomplete penetrance
b. incomplete dominance
c. codominance
d. overdominance
e. epistasis
Answer Key for Study Guide Questions
This answer key provides the answers to the exercises and chapter quiz for this chapter. Answers
in parentheses ( ) represent possible alternate answers to a problem, while answers marked with
an asterisk (*) indicate that the response to the question may vary.
5.1
1. Incomplete dominance
2. Overdominance
3. Codominance
4. Lethal allele
5. Sex-limited inheritance
5.2
1. e
2. d
3. c
4. a
5. b
5.3
1. temperature-sensitive
2. phenylalanine hydroxylase
3. phenylalanine
4. norm of reaction
5.4
1. It is an example of phenotypic blending since the heterozygote produces an
intermediate phenotype. However, the genotypes are not blended.
2. Phenotypic ratio is ½ red to ½ pink. The genotypic ratio is ½ CRCR to ½ CRCW.
3. O (donor); AB (recipient)
4. glycosyl transferases
5. heterozygote advantage
6. b
7. c
8. a
9. 1/2
10. 1/8
11. 1/16
5.5
1. 5--reductase
2. 5--dihydrotestosterone (DHT)
3. adrenal gland
4. heterozygous
5. 1/1 (100%)
6. 0%
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5.6
1. a
2. e
3. d
4. c
5. b
5.7
1. cystic fibrosis
2. chloride (Cl-)
3. mucus
4. vas deferens
5.8
1. Epistasis
2. Complementation
3. Gene redundancy
4. Paralogs
5. a. yes
b. yes
c. yes
6. Ratio is 15 triangular : 1 ovate
7. Ratio is 3 triangular : 1 ovate
Quiz
1. c
2. c
3. a
4. d
5. b
6. c
7. d
8. e
9. d
10. b
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