Download Chapter 4: Sex Determination and Sex Chromosomes

Chapter 4: Sex Determination and Sex Chromosomes
Student Learning Objectives
Upon completion of this chapter you should be able to:
1. Recognize the different methods of sex determination.
2. Understand the phenomenon of dosage compensation and know its molecular basis.
3. Differentiate between X-linked, Y-linked, and pseudoautosomal genes.
4. Distinguish between the various patterns of inheritance related to sex.
5. Understand the importance of Morgan’s work as a proof of the chromosome theory of
inheritance.
4.1 Mechanisms of Sex Determination Among Various Species
Overview
As humans, we are familiar with the fact that males have an X-Y combination of sex
chromosomes, and females possess an X-X combination. However, the X-Y system is not the
only mechanism of sex determination. Other organisms use various methods of identifying the
male and female of the species. In many organisms, sex is determined by the presence and
number of sex chromosomes. For example, birds utilize the Z-W system and some insects, like
grasshoppers, use the X-0 system (Refer to Figure 4.1). Indeed, other insects such as bees, wasps
and ants, use the haplodiploid mechanism of sex determination (males are haploid and females
diploid). In still other organisms, sex differences are determined by the environment: temperature
in alligators, and behavior in clownfish (Figure 4.3). This section ends with a discussion of
dioecious plants, in which individuals produce only male gametophytes or only female
gametophytes.
Outline of Key Terms
Autosomes
Sex determination
Sex chromosomes
Homogametic sex
Heterogametic sex
Haplodiploid system
Protandrous hermaphrodite
Dioecious
Focal Points



Sex determination via the presence of sex chromosomes (Figure 4.1)
Sex determination based on number of chromosome sets (Figure 4.2)
How environmental factors may determine sex (pages 75-76)
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Exercises and Problems
For questions 1 to 7, use the following answers.
a. X-Y system
b. X-O system
c. Z-W system
d. haplodiploid system
_____ 1.
In this system, the female is the heterogametic sex.
_____ 2.
Females in this system are the homogametic sex.
_____ 3.
Humans utilize this system of sex-determination.
_____ 4.
In this system, the female possesses two sex chromosomes, while the male only has
one sex chromosome.
_____ 5.
This form of sex determination is common in birds.
_____ 6.
In this system, sex is determined by the total chromosome number of the organism.
_____ 7.
This sex determination system is common in insects.
4.2 Dosage Compensation and X Inactivation in Mammals
Overview
For the most part, genes are responsible for the production of proteins, which in turn
determine the phenotype of the individual. In diploid organisms, autosomal genes are inherited in
pairs for all members of the species. However, for genes on the sex chromosomes, the sex of the
individual determines how many copies of the gene it possesses. Since too much of a gene
product can be just as detrimental as too little, there is a need to compensate for the dosage of
protein being produced. Table 4.1 provides the most common mechanisms by which this occurs.
This section begins with a discussion of dosage compensation in mammals, which occurs
by inactivation of one of the two X chromosomes in the female. The inactivated X chromosome
is called a Barr body (Figure 4.5a). The mechanism of X inactivation, also called the Lyon
hypothesis, is outlined in Figure 4.6. The section next covers the molecular mechanism of X
inactivation. Make sure at this point that you understand the roles of Xic and Xist (Figure 4.7).
Outline of Key Terms
Dosage compensation
X chromosome inactivation
Barr body
Lyon hypothesis
X inactivation center (Xic)
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Focal Points


Mechanisms of dosage compensation (Table 4.1, Figure 4.6)
The phases of X inactivation (Figure 4.7)
Exercises and Problems
For each of the following, complete the sentence with the appropriate word or phrase.
1.
2.
3.
4.
5.
6.
Dosage compensation in mammals occurs in the ___________ gender.
The inactivated X chromosome is called the _________ body.
The mechanism of X inactivation is also known as the __________ hypothesis.
X inactivation can lead to variegated phenotypes as is the case with _________ cats.
The ______ locus determines which X chromosome is to be inactivated.
The ______ gene encodes an RNA that coats the X chromosome and mediates its compaction.
4.3 Properties of the X and Y Chromosomes in Mammals
Overview
As we discussed in the first section of this chapter, sex determination in mammals is
determined by the presence of the Y chromosome, and specifically the Sry gene. This short
section examines the differences between the X and Y chromosomes of mammals. It focuses on
the genes found on these chromosomes. After studying this section, you should be able to
distinguish between X-linked, Y-linked, and pseudoautosomal genes.
Outline of Key Terms
Sex-linked genes
X-linked genes
Y-linked genes
Holandric
Pseudoautosomal genes
Pseudoautosomal inheritance
Focal Points

Comparison of the X and Y chromosomes (Figure 4.8)
Exercises and Problems
For each of the following, complete the sentence with the appropriate word or phrase.
1. Y-linked genes are also termed ___________ genes.
2. Genes found on only one sex chromosome but not both are called ___________ genes.
3. Genes found on both sex chromosomes are called __________ genes.
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4.4 Transmission Patterns of X-linked Genes
Overview
The first experimental proof of the chromosomal theory of inheritance was provided by
the American geneticist Thomas Hunt Morgan in his study of X-linked traits in Drosophila. The
experimental design of this proof is a classic example of how simple crosses were used to test the
developing theories of genetics in the early 20th century. You should examine Figure 4.9 and
understand how this elegant experiment provided proof that a gene for eye color was found on the
X chromosome. X-linked inheritance is also observed in mammals. This section ends with en
examination of Duchenne muscular dystrophy, which is an X-linked recessive disorder. Note that
for X-linked genes, reciprocal crosses yield different results (Figure 4.11b).
Outline of Key Terms
X-linked inheritance
Hemizygous
X-linked genes
X-linked alleles
Testcross
X-linked recessive pattern
Reciprocal cross
Focal Points


Morgan’s research which showed a connection between a genetic trait and the inheritance
of the X chromosome in Drosophila (pages 81-83)
Inheritance of X-linked recessive traits in mammals (Figures 4.10 and 4.11)
Exercises and Problems
1. What is the purpose of a testcross?
2. Explain how a reciprocal cross can be used to determine if a trait is sex-linked or autosomal.
For each of the following, complete the sentence with the appropriate word or phrase.
3.
4.
5.
6.
A male mammal or fruit fly is said to be ___________ for X-linked genes.
The genotype of a true-breeding red-eyed female fruit fly is designated ___________.
The genotype of a white-eyed male fruit fly is designated ___________.
Recessive X-linked disorders are more likely to occur in the __________ gender.
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Chapter Quiz
1. In which of the following is the male of the species the homogametic sex?
a. X-Y system
b. Z-W system
c. X-O system
d. haplodiploid system
e. the male is never homogametic
2. Temperature controls sex development in which of the following animals?
a. Birds
b. Bees
c. Fish
d. Grasshoppers
e. Alligators
3. Plant species in which individuals produce only male gametophytes or only female
gametophytes are called
a. isogamous.
b. protandrous.
c. dioecious.
d. hermaphroditic.
e. monogametic.
4. The Sry gene of humans displays which of the following patterns of inheritance?
a. X-linked
b. pseudoautosomal
c. autosomal
d. Y-linked
5. A human male with XXY chromosome composition will have how many Barr bodies?
a. 1
b. 2
c. 3
d. 0, because Barr bodies are only found in females.
6. The _______ genes encodes a(n) _______ that coats the X chromosome and mediates its
compaction and subsequent inactivation.
a. Xic
;
protein
b. Xic
;
RNA
c. Xist
;
protein
d. Xist
;
RNA
7. A reciprocal cross is used to determine which of the following?
a. if a trait is autosomal or sex-linked
b. the number of autosomal chromosomes
c. the dominance/recessive nature of a trait
d. the sex of the organism
e. none of the above
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8. Thoman Hunt Morgan’s experiment with white-eyed Drosophila provided proof of which of
the following?
a. law of independent assortment
b. chromosomal theory of inheritance
c. theory of natural selection
d. law of segregation
e. none of the above
9. Color-blindness is an X-linked recessive disorder. A color-blind man and a woman with
normal vision have a daughter who is color-blind. The most likely genotypes for the parents are:
a. Mother is XCXc and father is XcY
b. Mother is XcXc and father is XcY
c. Mother is XCXc and father is XCY
d. Mother is XcXc and father is XCY
10. Which of the following genotypes denotes a hemizygous individual?
a. XAXA
b. XAXa
c. XAY
d. Aa
e. ZZ
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.
4.1
1. c
2. a (b)
3. a
4. b
5. c
6. d
7. b (d)
4.2
1. female
2. Barr
3. Lyon
4. calico
5. Xic
6. Xist
4.3
1. holandric
2. sex-linked
3. pseudoautosomal
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4.4
1. The purpose of a testcross is to determine the genotype of an individual by crossing
the individual with a homozygous recessive individual.
2. In a reciprocal cross, if the sexes in the F1 generation differ for the trait being studied,
then the trait is sex-linked.
3. hemizygous
4. Xw+Xw+
5. XwY
6. male
Quiz
1. b
2. e
3. c
4. d
5. a
6. d
7. a
8. b
9. a
10. c
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