Page 137 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

Problems 129

33. In 1919, Calvin Bridges began studying an X-linked Section 4.7
recessive mutation causing eosin-colored eyes in 36. The following is a pedigree of a family in which a
Drosophila. Within an otherwise true-breeding rare form of color blindness is found (filled-in
culture of eosin-eyed flies, he noticed rare variants symbols). Indicate as much as you can about the
that had much lighter cream-colored eyes. By inter- genotypes of all the individuals in the pedigree.
crossing these variants, he was able to make a true-
breeding cream-eyed stock. Bridges now crossed I
males from this cream-eyed stock with true-breeding 1 2
wild-type females. All the F 1 progeny had red (wild- II 1 2 3 4
type) eyes. When F 1 flies were intercrossed, the F 2
progeny were 104 females with red eyes, 52 males III 1 2 3 4
with red eyes, 44 males with eosin eyes, and
14 males with cream eyes. Assume that these 37. Each of the four pedigrees that follow represents a
numbers represent an 8:4:3:1 ratio. human family within which a genetic disease is
a. Formulate a hypothesis to explain the F 1 and F 2 segregating. Affected individuals are indicated by
filled-in symbols. One of the diseases is transmitted
results, assigning phenotypes to all possible as an autosomal recessive condition, one as an
genotypes. X-linked recessive, one as an autosomal dominant,
b. What do you predict in the F 1 and F 2 genera- and one as an X-linked dominant. Assume all four
tions if the parental cross is between true- traits are rare in the population and completely
breeding eosin-eyed males and true-breeding penetrant.
cream-eyed females? a. Indicate which pedigree represents which mode of
c. What do you predict in the F 1 and F 2 generations inheritance, and explain how you know.
if the parental cross is between true-breeding b. For each pedigree, what would you tell the parents
eosin-eyed females and true-breeding cream- about the chance that their child (indicated by the
eyed males? hexagon shape) will have the condition?
34. In Drosophila, a cross was made between a yellow-
bodied male with vestigial (not fully developed) Pedigree 1
wings and a wild-type female (brown body). The F 1 1 2
generation consisted of wild-type males and wild-type
females. F 1 males and females were crossed, and the 1 2 3 4 5
F 2 progeny consisted of 16 yellow-bodied males with Pedigree 2 1 2
vestigial wings, 48 yellow-bodied males with normal
wings, 15 males with brown bodies and vestigial 1 2 3 4 5 6
wings, 49 wild-type males, 31 brown-bodied females
with vestigial wings, and 97 wild-type females. 1
Explain the inheritance of the two genes in question Pedigree 3 1 2
based on these results.
35. As we learned in this chapter, the white mutation of 1 2 3 4 5 6
Drosophila studied by Thomas Hunt Morgan is 1
X-linked and recessive to wild type. When true- Pedigree 4
breeding white-eyed males carrying this mutation 1 2
were crossed with true-breeding purple-eyed females,
all the F 1 progeny had wild-type (red) eyes. When the 1 2 3 4 5 6 7
F 1 progeny were intercrossed, the F 2 progeny emerged 1
in the ratio 3/8 wild-type females: 1/4 white-eyed 38. The pedigree that follows indicates the occurrence
males: 3/16 wild-type males: 1/8 purple-eyed females: of albinism in a group of Hopi Indians, among
1/16 purple-eyed males. whom the trait is unusually frequent. Assume that
a. Formulate a hypothesis to explain the inheritance the trait is fully penetrant (all individuals with a
of these eye colors. genotype that could give rise to albinism will display
b. Predict the F 1 and F 2 progeny if the parental cross this condition).
was reversed (that is, if the parental cross was a. Is albinism in this population caused by a recessive
between true-breeding white-eyed females and or a dominant allele?
true-breeding purple-eyed males). b. Is the gene sex-linked or autosomal?