Chapter 5: Extensions of Mendelian Inheritance

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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
    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


        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.   The heterozygote's phenotype is intermediate between those of the homozygotes.
_______    2.   The heterozygote has a trait that is more beneficial than either homozygote.
_______    3.   The heterozygote expresses both alleles simultaneously.
_______    4.   The allele has the potential to cause the death of an organism.
_______    5.   The trait occurs in only one of the two sexes.

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5.2 Dominant and Recessive Alleles


          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

Wild-type alleles                                      Dominant mutant alleles
   Genetic polymorphism                                    Gain-of-function mutations
Mutant alleles                                             Dominant negative mutations
                                                       Incomplete penetrance

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.

                                             - 47 -
5.3 Environmental Effects on Gene Expression


        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.   Coat color in the arctic fox is caused by a ______-______ allele.
2.   Phenylketonuria (PKU) is caused by a defect in the gene that encodes the enzyme _______.
3.   Individuals with PKU are placed on restricted diets that are free of _______.
4.   The term _______ refers to the effects of environmental variation on a phenotype.

5.4 Incomplete Dominance, Overdominance, and Codominance


         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.

Outline of Key Terms

Incomplete dominance
Overdominance                                         Codominance
    Heterozygote advantage                            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?

5.5 Sex-Influenced and Sex-Limited Inheritance


        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


         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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