Lecture 11: Sex Linkage
(Old Lecture 9)
1. Do males of all species have Y chromosomes? Explain your answer.
2. Briefly define each of the following and explain its significance to the study of genetics.
a. Autosome.
b. Homogametic.
c. Heterogametic.
d. Hemizygous
e. Sex-limited
f. Sex-influenced
3. How does the pattern of inheritance of sex-linked genes differ from that of autosomal genes:
a. for recessive alleles?
b. for dominant alleles?
4. 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?
5. 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.
6. How will the following differ in a ZZ/ZW system, compared to an XX/XY system?
a. Which will be the homogametic sex in each?
b. Which sex will exhibit hemizygous expression of recessive genes in each?
c. Would you expect large numbers of essential genes to be carried on the W chromosome? Relate your answer to the Y chromosome.
d. If a male is heterozygous for a Z-linked trait, how will it be expressed in his female progeny. Describe a comparable phenomenon in an XX/XY system.
7. 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 eye locus is on the X chromosome.
8. 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.
9. Why would you not expect to see a sex-linked dominant lethal in your collection of flies in question 8?
10. Distinguish among traits that are sex-linked, sex-limited, and sex-influenced.
11. What possible reasons exist for the scarcity of genes known to be carried on the human Y chromosome?
12. 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?
13. 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?
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