1. How do the XX/XY sex determination mechanisms differ between humans and Drosophila? How are they similar?
2. Do males of all species have Y chromosomes? Explain your answer.
3. Briefly define each of the following and explain its significance to the study of genetics.
a. Autosome.
b. Homogametic.
c. Heterogametic.
d. SRY (Sry)
e. Sxl
f. Pseudoautosomal.
g. Hemizygous
4. The human gene that codes for the testosterone receptor is carried on the human X chromosome.
a. What biological role does the testosterone receptor play?
b. What are the developmental consequences in an XY human who is hemizygous for a defective testosterone receptor gene?
c. Why is one unlikely to encounter human females who are homozygous for defective testosterone receptors?
d. What aspects of normal female development are absent in individuals with testicular feminization and why?
e. Why is it not possible to reverse testicular feminization with testosterone injections?
5. What is a gynandromorph and how is it formed?
6. What is the difference between an X chromosome and a Z chromosome?
7. Can the gene that codes for the testosterone receptor be described as a sex-determining gene? Defend your answer.
8. Why is the designation sex lethal (Sxl) not an accurate description of the role of the gene or of its loss of function mutation?
9. Distinguish between monoecious and dioecious in a manner that makes it clear you know what each is and how they differ.
10. How does the pattern of inheritance of sex-linked genes differ from that of autosomal genes:
a. for recessive alleles?
b. for dominant alleles?
11. a and b are two genes located close together on the X chromosome in Drosophila, such that crossing over between them is a rare event that can be ignored for purposes of this problem. Assume that in both cases, wild-type is fully dominant. A female with the genotype aab+b+ is mated with a male of the genotype a+b.
a. What will be the genotype and phenotype of male F1 progeny?
b. What will be the genotype and phenotype of the female F1 progeny?
c. What will be the genotype and phenotype of male F2 progeny?
d. What will be the genotype and phenotype of the female F2 progeny?
12. Same problem as 4, except assume that a and b are located so far apart on the X chromosome that there is enough crossing over so that they appear to assort independently. However, crossing over does not occur in male Drosophila (and could not in this case anyhow, since there is only one X chromosome). Identify as many classes of progeny as you need to answer the question fully.
13. Starting with a white-eyed male Drosophila and a wild-type female, describe the series of crosses that you would have to do to generate a true-breeding population. The white-eyed locus is on the X chromosome.
14. You have collected a number of male Drosophila that exhibit numerous different mutant phenotypes. You also have access to a colony of wild-type Drosophila. Summarize the steps you would have to do to verify that you had a mutant from each of the following categories in your collection. Assume that in every case the mutant flies carry only as many mutant alleles as they need to exhibit the phenotype.
a. Sex-linked recessive.
b. Sex-linked dominant.
c. Autosomal recessive.
d. Autosomal dominant that is not lethal when homozygous.
e. Autosomal dominant that is lethal when homozygous.
15. Why would you not expect to see a sex-linked dominant lethal in your collection of flies in question 8?
16. What possible reasons exist for the scarcity of genes known to be carried on the human Y chromosome?
17. A woman who is homozygous for a recessive trait (t) marries a man who is not a carrier of that trait. They have one son. What additional information would you need to determine whether the gene for t is autosomal or sex linked?
18. A color-blind man marries a woman with normal vision. They have a color-blind daughter.
a. What is the genotype of the mother?
b. Is the information presented in this problem enough for you to conclude that color-blindness is carried on the X-chromosome if you had no prior information about it? Justify your answer.
c. Would the answer to part b have been different if the color-blind child were a son?
19. A couple who are both phenotypically normal have two sons. One has Duchenne muscular dystrophy and normal color vision. The other is color blind and free of muscle disease.
a. Based on what is known about the inheritance of these two genetically determined phenotypes, what is the most likely genotype of the father?
b. What is the most likely genotype of the mother.
c. What is the most likely genotype of the mother's father (the maternal grandfather of the afflicted children)?
d. What is the most likely genotype of the mother's mother (the maternal grandmother of the afflicted children)?
e. The mother has two brothers, each of whom has a different phenotype. What are their most likely phenotypes.
20. A woman is heterozygous for Duchenne muscular dystrophy. She has two daughters who are phenotypically normal. They both marry phenotypically normal men. One has a daughter, the other has a son. Those two grandchildren, who are first cousins, marry and have children. Are those children at any greater risk of developing Duchenne muscular dystrophy than if the cousins had each married an unrelated person and had children. Explain the reasoning behind your answer.
21. Would your answer to question 20 have been any different if the woman had been heterozygous for Tay-Sachs disease, which is autosomal. Explain the reasoning behind your answer.
Go to Review Questions for the Following Lecture
Return to Index of Review Questions
Go to Review Questions for the Previous Lecture
Return to Lecture 28