Page 180 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

172 Chapter 5 Linkage, Recombination, and the Mapping of Genes on Chromosomes

Assume for the moment that the same is true in hu- a. Are the a and w loci linked? If so, how far apart
mans. The disease sickle-cell anemia is the result of are they?
homozygosity for a particular mutation in the b. What was the genotype of the blue smooth parent?
β-globin gene. Include the chromosome arrangement of alleles.
a. A son is born to an albino man and a woman with c. If a plant grown from a blue wrinkled progeny seed
sickle-cell anemia. What kinds of gametes will the is crossed to a plant grown from a yellow smooth
son form, and in what proportions? F 1 seed, what kinds of kernels would be expected,
b. A daughter is born to a normal man and a woman and in what proportions?
who has both albinism and sickle-cell anemia. 13. If the a and b loci are 40 cM apart and an AA BB indi-
What kinds of gametes will the daughter form, vidual and an aa bb individual mate:
and in what proportions? a. What gametes will the F 1 individuals produce, and
c. If the son in part (a) grows up and marries the in what proportions? What phenotypic classes in
daughter in part (b), what is the probability that a what proportions are expected in the F 2 generation
child of theirs will be an albino with sickle-cell (assuming complete dominance for both genes)?
anemia? b. If the original cross was AA bb × aa BB, what
10. In a particular human family, John and his mother gametic proportions would emerge from the F 1 ?
both have brachydactyly (a rare autosomal domi- What would be the result in the F 2 generation?
nant allele causing short fingers). John’s father has 14. Write the number of different kinds of phenotypes,
Huntington disease (another rare autosomal domi- excluding sex, you would see among a large number
nant allele). John’s wife is phenotypically normal of progeny from an F 1 mating between individuals of
and is pregnant. Two-thirds of people who inherit identical genotype that are heterozygous for one or
the Huntington (HD) allele show symptoms by age two genes (that is, Aa or Aa Bb) as indicated. No gene
50, and John is 50 and has no symptoms. interactions means that the phenotype determined by
Brachydactyly is 90% penetrant. one gene is not influenced by the genotype of the
a. What are the genotypes of John’s parents? other gene.
b. What are the possible genotypes for John? How a. One gene; A completely dominant to a.
likely is John to have each of these genotypes? b. One gene; A and a codominant.
c. What is the probability the child will express both
brachydactyly and Huntington disease by age 50 if c. One gene; A incompletely dominant to a.
the two genes are unlinked? d. Two unlinked genes; no gene interactions; A
d. How will your answer to part (c) change if instead completely dominant to a, and B completely
these two loci are 20 m.u. apart? dominant to b.
11. Albino rabbits (lacking pigment) are homozygous for e. Two genes, 10 m.u. apart; no gene interactions; A
completely dominant to a, and B completely domi-
the recessive c allele (C allows pigment formation).
Rabbits homozygous for the recessive b allele make nant to b.
brown pigment, while those with at least one copy of f. Two unlinked genes; no gene interactions; A and
B make black pigment. True-breeding brown rabbits a codominant, and B incompletely dominant to b.
were crossed to albinos, which were also BB. F 1 rab- g. Two genes, 10 m.u. apart; A completely dominant
bits, which were all black, were crossed to the double to a, and B completely dominant to b; and with
recessive (bb cc). The progeny obtained were 34 recessive epistasis between aa and the alleles of
black, 66 brown, and 100 albino. gene B.
a. What phenotypic proportions would have been h. Two unlinked duplicated genes (that is, A and B
expected if the b and c loci were unlinked? perform the same function); A and B completely
b. How far apart are the two loci? dominant to a and b, respectively.
12. In corn, the allele A allows the deposition of anthocy- i. Two genes, 0 m.u. apart; no gene interactions; A
anin (blue) pigment in the kernels (seeds), while aa completely dominant to a, and B completely domi-
plants have yellow kernels. At a second gene, W– nant to b. (Two possible answers exist.)
produces smooth kernels, while ww kernels are wrin- 15. A DNA variant has been found linked to a rare auto-
kled. A plant with blue smooth kernels was crossed somal dominant disease in humans and can thus be
to a plant with yellow wrinkled kernels. The progeny used as a marker to follow inheritance of the disease
consisted of 1447 blue smooth, 169 blue wrinkled, allele. In an informative family (in which one parent
186 yellow smooth, and 1510 yellow wrinkled. is heterozygous for both the disease allele and the