5 Chapter Answers to Homework Problems.
1. Questions and Problems, 5.1 What are the genetic differences between male- and female-determining sperm in animals with heterogametic males? Ans: The male-determining sperm carries a Y chromosome; the female-determining sperm carries an X chromosome. 2. Questions and Problems, 5.2 A male with singed bristles appeared in a culture of Drosophila. How would you determine if this unusual phenotype was due to an X-linked mutation? Ans: Cross the singed male to wild-type females, and then intercross the offspring. If the singed bristle phenotype is due to an X-linked mutation, approximately half the F2 males, but none of the F2 females, will show it. 5. Questions and Problems, 5.5 What are the sexual phenotypes of the following genotypes in Drosophila: XX, XY, XXY, XXX, XO? Ans: XX is female, XY is male, XXY is female, XXX is female (but barely viable), XO is male (but sterile). 7. Questions and Problems, 5.7 If a father and son both have defective color vision, is it likely that the son inherited the trait from his father? Ans: No. Defective color vision is caused by an X-linked mutation. The sons X chromosome came from his mother, not his father. 9. Questions and Problems, 5.9 A man with X-linked color blindness marries a woman with no history of color blindness in her family. The daughter of this couple marries a normal man, and their daughter also marries a normal man. What is the chance that this last couple will have a child with color blindness? If this couple has already had a child with color blindness, what is the chance that their next child will be color blind? Ans: The risk for the child is P(mother is C/c) x P(mother transmits c) x P(child is male) = (1/2) x (1/2) x (1/2) = 1/8; if the couple has already had a child with color blindness, P(mother is C/c) = 1, and the risk for each subsequent child is 1/4. 10. Questions and Problems, 5.10 A man who has color blindness and type O blood has children with a woman who has normal color vision and type AB blood. The womans father had color blindness. Color blindness is determined by an Xlinked gene, and blood type is determined by an autosomal gene. (a) What are the genotypes of the man and the woman?
(b) What proportion of their children will have blindness color and type B blood? (c) What proportion of their children will have color blindness and type A blood? (d) What proportion of their children will be color blind and have type AB blood? Ans: (a) The man is XcY ii; the woman is X+ Xc IA IB. (b) Probability color blind = 1/2; probability type B blood = 1/2; combined probability = (1/2) (1/2) = 1/4. (c) Probability color blind = 1/2; probability type A blood = 1/2; combined probability (1/2) (1/2) = 1/4. (d) 0. 15. Suppose that a mutation occurred in the SRY gene on the human Y chromosome, knocking out its ability to produce the testis-determining factor. Predict the phenotype of an individual who carried this mutation and a normal X chromosome. Ans: Female. 16. Questions and Problems, 5.16 A woman carries the testicular feminization mutation (tfm) on one of her X chromosomes; the other X carries the wild-type allele (Tfm). If the woman marries a normal man, what fraction of her children will be phenotypically female? Of these, what fraction will be fertile? Ans: Three-fourths will be phenotypically female (genotypically tfm/Tfm, Tfm/Tfm, or tfm/Y). Among the females, 2/3 (tfm/Tfmm and Tfm/Tfm) will be fertile; the tfm/Y females will be sterile. 17. Questions and Problems, 5.17 Would a human with two X chromosomes and a Y chromosome be male or female? Ans: Male. 19. Questions and Problems, 5.19 Predict the sex of Drosophila with the following chromosome compositions (A = haploid set of autosomes): (a) 4X 4A; (b) 3X 4A; (c) 2X 3A; (d) 1X 3A; (e) 2X 2A; (f) 1X 2A. Ans: (a) Female; (b) intersex; (c) intersex; (d) male: (e) female; (f) male. 21. Questions and Problems, 5.21 A Drosophila male carrying a recessive X-linked mutation for yellow body is mated to a homozygous wild-type female with gray body. The daughters of this mating all have uniformly gray bodies. Why arent their bodies a mosaic of yellow and gray patches? Ans: Drosophila does not achieve dosage compensation by inactivating one of the X chromosomes in females.
22. Questions and Problems, 5.22 What is the maximum number of Barr bodies in the nuclei of human cells with the following chromosome compositions: (a) XY; (b) XX; (c) XXY; (d) XXX; (e) XXXX; (f) XYY? Ans: (a) Zero; (b) one; (c) one; (d) two; (e) three; (f) zero.