Phenotypes of progeny F2 of Wild Cinna- Choco- late line type Black mon Albino 1 87 32 39 2 62 18 96 30 41 164 4 287 86 92 29 (Adapted from A. M. Srb, R. D. Owen, and R. S. Edgar, General Geneties, 2nd ed. W. H. Freeman and Company, 1965.)
. Many kinds of wild animals have the agouti coloring pattern, in which each hair has a yellow band around it.
a. Black mice and other black animals do not have the
yellow band; each of their hairs is all black. This absence
of wild agouti pattern is called nonagouti. When mice of
a true-breeding agouti line are crossed with nonagoutis,
the F1 is all agouti and the F2 has a 3 :1 ratio of agoutis to
nonagoutis. Diagram this cross, letting A represent the
allele responsible for the agouti phenotype and a,
nonagouti. Show the
parents, their gametes, the F1, their gametes, and the F2.
b. Another inherited color deviation in mice substitutes
brown for the black color in the wild-type hair. Such
brown-agouti mice are called cinnamons. When wildtype mice are crossed with cinnamons, all of the F1 are
wild type and the F2 has a 3 :1 ratio of wild type to
cinnamon. Diagram this cross as in part a, letting B stand
for the wild-type black allele and b stand for the
cinnamon brown allele.
c. When mice of a true-breeding cinnamon line are
crossed with mice of a true-breeding nonagouti (black)
line, all of the F1 are wild type. Use a genetic diagram to
explain this result.
d. In the F2 of the cross in part c, a fourth color called
chocolate appears in addition to the parental cinnamon
and nonagouti and the wild type of the F1. Chocolate
mice have a solid, rich brown color. What is the genetic
constitution of the chocolates?
e. Assuming that the A/a and B/b allelic pairs assort
independently of each other, what do you expect to be
the relative frequencies of the four color types in the F2
described in part d? Diagram the cross of parts c and d,
showing phenotypes and genotypes (including gametes).
f. What phenotypes would be observed in what
proportions in the progeny of a backcross of F1 mice
from part c with the cinnamon parental stock? With
the nonagouti (black) parental stock? Diagram these
backcrosses.
g. Diagram a testcross for the F1 of part c. What colors
would result and in what proportions?
h. Albino (pink-eyed white) mice are homozygous for
the recessive member of an allelic pair C/c, which assorts
independently of the A/a and B/b pairs. Suppose that
you have four different highly inbred (and therefore
presumably homozygous) albino lines. You cross each of
these lines with a true-breeding wild-type line, and you
raise a large F2 progeny from each cross. What genotypes
for the albino lines can you deduce from the following F2
phenotypes?(Adapted from A. M. Srb, R. D. Owen, and R. S. Edgar,
General Genetics, 2nd ed. W. H. Freeman and Company,
1965.)
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