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 \(F_{1}\) is all
agouti and the \(F_{2}\) 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 phenotypes and genotypes of the parents, their
gametes, the \(F_{1}\), their gametes, and the \(F_{2}\)
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 \(\mathrm{F}_{1}\) are wild
type and the \(\mathrm{F}_{2}\) 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 \(F_{1}\) are wild type. Use a
genetic diagram to explain this result.
d. In the \(F_{2}\) 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 \(\mathrm{F}_{1}\). 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 \(\mathrm{F}_{2}\) 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 \(\mathrm{F}_{1}\) mice from part \(c\) with the cinnamon parental
stock? With the nonagouti (black) parental stock? Diagram these backcrosses.
g. Diagram a testcross for the \(\mathrm{F}_{1}\) 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 \(\mathrm{F}_{2}\) progeny
from each cross. What genotypes for the albino lines can you deduce from the
following \(\mathrm{F}_{2}\) phenotypes?
$$\begin{array}{cccccc}
& {4}{c} {\text {Phenotypes of progeny}} \\
{ 2 - 5 } \mathrm{F}_{2} \text { of }
\text { line } & \begin{array}{c}
\text { Wild } \\
\text { type }
\end{array} & \text { Black } & \begin{array}{c}
\text { Cinna- } \\
\text { mon }
\end{array} & \begin{array}{c}
\text { Choco- } \\
\text { late }
\end{array} & \text { Albino } \\
\hline 1 & 87 & 0 & 32 & 0 & 39 \\
2 & 62 & 0 & 0 & 0 & 18 \\
3 & 96 & 30 & 0 & 0 & 41 \\
4 & 287 & 86 & 92 & 29 & 164 \\
\hline
\end{array}$$
(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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