In rats, several independently assorting autosomal genes affect coat color. Gene

Question

In rats, several independently assorting autosomal genes affect coat color. Gene A controls the distribution of yellow pigment in hair, and gene B causes black pigmentation. The two genes interact as follows: A–B– (gray), A–bb (yellow), aaB– (black), and aabb (cream). These genotypes are only expressed in the presence of the dominant allele of a third gene, C; rats with genotype cc are albino.

a.    Deduce the genotype of the albino mice, to the extent that is possible, in the following table. Explain your answers.

True-breeding parents

F1

F2 offspring

Gray x albino

All gray

9/16 gray : 3/16 yellow : 4/16 albino

b.    Deduce the genotype and phenotype of each parent in the following table. Explain your answers.

Cross

Numbers of offspring

Parent A x Parent B

135 gray, 83 albino, 47 yellow, 44 black, 16 cream

c.     A gray-colored rat is mated with one that is yellow. The offspring include an albino rat and a cream-colored rat. Diagram this cross. Be sure to include the Punnett square and the phenotypic ratio in the offspring.

True-breeding parents

F1

F2 offspring

Gray x albino

All gray

9/16 gray : 3/16 yellow : 4/16 albino

Explanation / Answer

In rats, several independently assorting autosomal genes affect coat color. Gene A controls the distribution of yellow pigment in hair, and gene B causes black pigmentation. The two genes interact as follows: A–B– (gray), A–bb (yellow), aaB– (black), and aabb (cream). These genotypes are only expressed in the presence of the dominant allele of a third gene, C; rats with genotype cc are albino.

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