Essentials of Genetics

William S. Klug, Michael R. Cummings, Charlotte A. Spencer

Chapter 5

Sex Determination and Sex Chromosomes - all with Video Answers

Educators

Chapter Questions

Problem 1

In this chapter, we have focused on sex differentiation, sex chromosomes, and genetic mechanisms involved in sex determination. At the same time, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, you should answer the following fundamental questions:
(a) How do we know that in humans the X chromosomes play no role in sex determination, while the Y chromosome causes maleness and its absence causes femaleness?
(b) How did we originally (in the late 1940 s) analyze the sex ratio at conception in humans, and how has our approach to studying this issue changed in $2015 ?$
(c) How do we know that X chromosomal inactivation of either the paternal or maternal homolog is a random event during early development in mammalian females?
(d) How do we know that Drosophila utilizes a different sexdetermination mechanism than mammals, even though it has the same sex-chromosome compositions in males and females?

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 2

Review the Chapter Concepts list on
p. $83 .$ These all center on sex determination or the expression of genes encoded on sex chromosomes. Write a short essay that discusses sex chromosomes as they contrast with autosomes.

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 3

As related to sex determination, what is meant by
(a) homomorphic and heteromorphic chromosomes; and
(b) homogametic sex and heterogametic sex?

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 4

Distinguish between the concepts of sex determination and sexual differentiation.

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 5

Distinguish between the Protenor and Lygaeus modes of sex determination.

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 6

Describe the major difference between sex determination in Drosophila and in humans.

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 7

How do mammals, including humans, solve the "dosage problem" caused by the presence of an $\mathrm{X}$ and $\mathrm{Y}$ chromosome in one sex and two X chromosomes in the other sex?

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 8

What specific observations (evidence) support the conclusions about sex determination in Drosophila and humans?

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 9

Describe how nondisjunction in human female gametes can give rise to Klinefelter and Turner syndrome offspring following fertilization by a normal male gamete.

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 10

An insect species is discovered in which the heterogametic sex is unknown. An X-linked recessive mutation for reduced wing (rw) is discovered. Contrast the $F_{1}$ and $F_{2}$ generations from a cross between a female with reduced wings and a male with normalsized wings when
(a) the female is the heterogametic sex.
(b) the male is the heterogametic sex.

Raechel Tittor

Numerade Educator

Problem 11

Given your answers to Problem $10,$ is it possible to distinguish between the Protenor and Lygaeus mode of sex determination based on the outcome of these crosses?

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 12

When cows have twin calves of unlike sex (fraternal twins), the female twin is usually sterile and has masculinized reproductive organs. This calf is referred to as a freemartin. In cows, twins may share a common placenta and thus fetal circulation. Predict why a freemartin develops.

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 13

An attached-X female fly, XXY (see the Insights and Solutions box), expresses the recessive X-linked white-eye phenotype. It is crossed to a male fly that expresses the X-linked recessive miniature wing phenotype. Determine the outcome of this cross in terms of sex, eye color, and wing size of the offspring.

Tom Comey

Numerade Educator

Problem 14

Assume that on rare occasions the attached $X$ chromosomes in female gametes become unattached. Based on the parental phenotypes in Problem $13,$ what outcomes in the $\mathrm{F}_{1}$ generation would indicate that this has occurred during female meiosis?

Joanna Quigley

Numerade Educator

Problem 15

It is believed that any male-determining genes contained on the Y chromosome in humans are not located in the limited region that synapses with the X chromosome during meiosis. What might be the outcome if such genes were located in this region?

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 16

What is a Barr body, and where is it found in a cell?

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 17

Indicate the expected number of Barr bodies in interphase cells of individuals with Klinefelter syndrome; Turner syndrome; and karyotypes $47, \mathrm{XYY}, 47, \mathrm{XXX},$ and $48, \mathrm{XXXX}$

Zachary Papazian

Zachary Papazian

Numerade Educator

Problem 18

Define the Lyon hypothesis

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 19

Can the Lyon hypothesis be tested in a human female who is homozygous for one allele of the X-linked G6PD gene? Why, or why not?

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 20

Predict the potential effect of the Lyon hypothesis on the retina of a human female heterozygous for the X-linked red-green colorblindness trait.

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 21

Cat breeders are aware that kittens expressing the X-linked calico coat pattern and tortoiseshell pattern (Figure 5.6 ) are almost invariably females. Why?

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 22

What does the apparent need for dosage compensation mechanisms suggest about the expression of genetic information in normal diploid individuals?

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 23

In mice, the Sry gene (see Section 5.2) is located on the Y chromosome very close to one of the pseudoautosomal regions that pairs with the X chromosome during male meiosis. Given this information, propose a model to explain the generation of unusual males who have two X chromosomes (with an Sry-containing piece of the Y chromosome attached to one X chromosome).

Rabeya Zahid

Numerade Educator

Problem 24

The genes encoding the red-and green-color-detecting proteins of the human eye are located next to one another on the X chromosome and probably evolved from a common ancestral pigment gene. The two proteins demonstrate 76 percent homology in their amino acid sequences. A normal-visioned woman with both genes on each of her two X chromosomes has a redcolor-blind son who was shown to have one copy of the greendetecting gene and no copies of the red-detecting gene. Devise an explanation for these observations at the chromosomal level (involving meiosis).

Pronoy Sinha

Numerade Educator

Problem 25

In mice, the X-linked dominant mutation Testicular feminization (Tfm) eliminates the normal response to the testicular hormone testosterone during sexual differentiation. An XY mouse bearing the $T f m$ allele on the $X$ chromosome develops testes, but no further male differentiation occurs-the external genitalia of such an animal are female. From this information, what might you conclude about the role of the Tfm gene product and the X and $Y$ chromosomes in sex determination and sexual differentiation in mammals? Can you devise an experiment, assuming you can "genetically engineer" the chromosomes of mice, to test and confirm your explanation?

Prashant Bana

Numerade Educator

Problem 26

In chickens, a key gene involved in sex determination has recently been identified. Called $D M R T 1$, it is located on the $Z$ chromosome and is absent on the W chromosome. Like SRY in humans, it is male determining. Unlike $S R Y$ in humans, however, female chickens (ZW) have a single copy while males (ZZ) have two copies of the gene. Nevertheless, it is transcribed only in the developing testis. Working in the laboratory of Andrew sinclair (a co-discoverer of the human $S R Y$ gene), Craig Smith and colleagues were able to "knock down" expression of $D M R T 1$ in $Z Z$ embryos using RNA interference techniques (see Chapter 16 ). In such cases, the developing gonads look more like ovaries than testes [Nature 461: 267 (2009)]. What conclusions can you draw about the role that the DMRT1 gene plays in chickens in contrast to the role the SRY gene plays in humans?

John Barone

Numerade Educator

Problem 27

Shown here are graphs that plot the percentage of fertilized eggs containing males against the atmospheric temperature during early development in
(a) snapping turtles and
(b) most lizards. Interpret these data as they relate to the effect of
temperature on sex determination.

Dennis Howard

Numerade Educator