Concepts of Genetics

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

Chapter 7

Sex Determination and Sex Chromosomes - all with Video Answers

Educators

Chapter Questions

Problem 1

In this chapter, we 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, what answers would you propose to the following fundamental questions?
(a) How do we know that specific genes in maize play a role in sexual differentiation?
(b) How do we know whether or not a heteromorphic chromosome such as the Y chromosome plays a crucial role in the determination of sex?
(c) How do we know that in humans the X chromosomes play no role in human sex determination, while the Y chromosome causes maleness and its absence causes femaleness?
(d) How did we learn that, although the sex ratio at birth in humans favors males slightly, the sex ratio at conception favors them much more?
(e) 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?
(f) 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?

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 2

Review the Chapter Concepts list on $\mathrm{p}$. $198 .$ These all center around 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

Contrast the life cycle of a plant such as Zea mays with an animal such as $C$. elegans.

Prashant Bana

Numerade Educator

Problem 4

Distinguish between the terms homomorphic and heteromorphic chromosomes, and between isogamous and heterogamous organisms.

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 5

Distinguish between the concepts of sexual differentiation and sex determination.

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 6

Describe how temperature variation controls sex determination in crocodiles.

Carlene Jimenez

Carlene Jimenez

Numerade Educator

Problem 7

Describe the major differences between $\mathrm{XO}$ individuals in Drosophila and those in humans.

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 8

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 9

The phenotype of an early-stage human embryo is considered sexually indifferent. Explain why this is so even though the embryo's genotypic sex is already fixed.

Asma Venkitta

Numerade Educator

Problem 10

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

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 11

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 12

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

Qudsiya Anis

Numerade Educator

Problem 13

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 14

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

Bryan Valdivia

Numerade Educator

Problem 15

Assume that on rare occasions the attached $X$ chromosomes in female gametes become unattached. Based on the parental phenotypes in Problem $14,$ what outcomes in the $\mathrm{F}_{1}$ generation would indicate that this has occurred during female meiosis?

Ayushi Balan

Numerade Educator

Problem 16

It has been suggested that any male-determining genes contained on the $Y$ chromosome in humans cannot be 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 17

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

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 18

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 19

Define the Lyon hypothesis.

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 20

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 21

A cross is made between a female calico cat and a male cat having the gene for black fur on his $X$ chromosome. What fraction of the offspring would one expect to be calico?

Grant Castaneda

Grant Castaneda

Numerade Educator

Problem 22

Under what circumstances can a male cat exhibit a tortoise-
shell coat pattern?

Breanna Kloczkowski

Breanna Kloczkowski

Numerade Educator

Problem 23

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 24

How does $X$ chromosome dosage compensation in Drosophila differ from that process in humans?

Dennis Howard

Numerade Educator

Problem 25

What type of evidence supports the conclusion that the primary sex ratio in humans is much higher than the secondary sex ratio?

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 26

Devise as many hypotheses as you can that might explain why so many more human male conceptions than human female conceptions occur.

Noah Boudrie

Numerade Educator

Problem 27

A color-blind, chromatin-positive male child (one Barr body) has a maternal grandfather who was color blind. The boy's mother and father are phenotypically normal. Construct and support (using appropriately labeled diagrams) a rationale whereby the chromosomal and genetic attributes of the chromatin-positive male are fully explained.

Sulav Pokhrel

Numerade Educator

Problem 28

In Drosophila, an individual female fly was observed to be of the XXY chromosome complement (normal autosomal complement) and to have white eyes as contrasted with the normal red eye color of wild type. The female's father had red eyes, and the mother had white eyes. Knowing that white eyes are X-linked and recessive, present an explanation for the genetic and chromosomal constitution of the XXY, white-eyed individual. It is important that you state in which parent and at what stage the chromosomal event occurred that caused the genetic and cytogenetic abnormality.

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 29

What is the role of the enzyme aromatase in sexual differentiation in reptiles?

Sam Limsuwannarot

Sam Limsuwannarot

Numerade Educator

Problem 30

In the wasp Bracon hebetor, a form of parthenogenesis (the development of unfertilized eggs into progeny) resulting in haploid organisms is not uncommon. All haploids are males. When offspring arise from fertilization, females almost invariably result. P. W. Whiting has shown that an X-linked gene with nine multiple alleles $\left(X_{a}, X_{b},$ etc.) controls sex determi- \right. nation. Any homozygous or hemizygous condition results in males, and any heterozygous condition results in females. If an $X_{a} / X_{b}$ female mates with an $X_{a}$ male and lays 50 percent fertilized and 50 percent unfertilized eggs, what proportion of male and female offspring will result?

Dennis Howard

Numerade Educator

Problem 31

The echidna has 27 pairs of autosomes and no less than nine sex chromosomes $\left(\mathrm{X}_{1} \mathrm{X}_{2} \mathrm{X}_{3} \mathrm{X}_{4} \mathrm{X}_{5} \text { and } \mathrm{Y}_{1} \mathrm{Y}_{2} \mathrm{Y}_{3} \mathrm{Y}_{4}\right) .$ Male echidnas
have the karyotype $63, \mathrm{X}_{1} \mathrm{Y}_{1} \mathrm{X}_{2} \mathrm{Y}_{2} \mathrm{X}_{3} \mathrm{Y}_{3} \mathrm{X}_{4} \mathrm{Y}_{4} \mathrm{X}_{5},$ while females are
$64, \mathrm{X}_{1} \mathrm{X}_{1} \mathrm{X}_{2} \mathrm{X}_{2} \mathrm{X}_{3} \mathrm{X}_{3} \mathrm{X}_{4} \mathrm{X}_{4} \mathrm{X}_{5} \mathrm{X}_{5}$. The nine sex chromosomes link to
form a chain during meiosis, which leads to disjunction of the $\mathrm{X}_{1} \mathrm{X}_{2} \mathrm{X}_{3} \mathrm{X}_{4} \mathrm{X}_{5}$ and $\mathrm{Y}_{1} \mathrm{Y}_{2} \mathrm{Y}_{3} \mathrm{Y}_{4}$ gametes. Several genes from the
human X chromosome, including G6PD, GDX, F9, AR, and $\mathrm{MCF} 2,$ map to the $\mathrm{X}_{1}$ of echidna. However, no equivalent of the SRY has been found, and although several of the sex chromosomes contain genes orthologous to those on avian $Z$, the significance of this is unknown. Based on what you have learned in this chapter, speculate on the possible mechanisms of sex determination in the echidna. How would you test your theories?

Khalida Dawar

Numerade Educator

Problem 32

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 $T f m$ gene product and the $\mathrm{X}$ and $\mathrm{Y}$ 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 33

When the cloned cat Carbon Copy (CC) was born (see the Now Solve This question on $\mathrm{p} .211$ ), she had black patches and white patches, but completely lacked any orange patches. The knowledgeable students of genetics were not surprised at this outcome. Starting with the somatic ovarian cell used as the source of the nucleus in the cloning process, explain how this outcome occurred.

Bryan Lynn

Numerade Educator

Problem 34

In a number of organisms, including Drosophila and butterflies, genes that alter the sex ratio have been described. In the pest species Musca domesticus (the house fly), Aedes aegypti (the mosquito that is the vector for yellow fever), and Culex pipiens (the mosquito vector for filariasis and some viral dis-
eases), scientists are especially interested in such genes. Sex in Culex is determined by a single gene pair, $M m$ being male and $m m$ being female. Males homozygous for the recessive gene $d d$ never produce many female offspring. The $d d$ combination in males causes fragmentation of the $m$ -bearing dyad during the first meiotic division, hence its failure to complete spermatogenesis.
(a) Account for this sex-ratio distortion by drawing labeled chromosome arrangements in primary and secondary spermatocytes for each of the following genotypes: $M m D d$ and $M m d d .$ How do meiotic products differ between $D d$ and $d d$ genotypes? Note that the diploid chromosome number is 6 in Culex pipiens and both $D$ and $M$ loci are linked on the same chromosome.
(b) How might a sex-ratio distorter such as $d d$ be used to control pest population numbers?

Bryan Lynn

Numerade Educator

Problem 35

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 $S R Y$ 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 17 ). 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 $D M R T 1$ gene plays in chickens in contrast to the role the SRY gene plays in humans?

John Barone

Numerade Educator

Problem 36

The paradigm in vertebrates is that, once sex determination occurs and testes or ovaries are formed, secondary sexual differentiation (male vs. female characteristics) is dependent on male or female hormones that are produced. Recently, D. Zhao and colleagues studied three chickens that were bilateral gynandromorphs, with the right side of the body being clearly female and the left side of the body clearly male [Nature 464 :
$237(2010)] .$ Propose experimental questions that can be investigated using these chickens to test this paradigm. What alternative interpretation contrasts with the paradigm?

Aadit Sharma

Numerade Educator