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Mendelian Inheritance

1.) Fur color in rabbits is determined by a single gene locus for which there are four alleles. Four phenotypes are possible: black, Chinchilla (gray color causes by white hairs with black tips), Himalayan (white with black patches on extremities), and white. The black allele (C) is dominant over all other alleles, the Chinchilla allele (C^ch) is dominant over Himalayan (C^h), and the white allele (c) is recessive to all others.

a.) A black rabbit is crossed with a Himalayan, and F1 consists of a ratio of 2 black to 2 Chinchilla. Can you determine the genotypes of the parents?

b.) A second cross was done between a black rabbit and a Chincilla. The F1 contained a ratio of 2 black to 1 Chinchilla to 1 Himalayan. Can you determine the genotypes of the parents of this cross?

2.) In Labrador retriever dogs, the dominant gene B determines black coat color and bb produces brown. A separate gene E, however, shows dominant epistasis over the B and b alleles, resulting in a "golden" coat color. The recessive e allows expression of B and b. A breeder wants to know the genotypes of her three dogs, so she breeds them and makes note of the offspring of several litters. Determine the genotypes of the three dogs.

a.) golden female (Dog 1) x golden male (Dog 2)
offspring: 7 golden, 1 black, 1 brown

b.) black female (Dog 3) x golden male (Dog 2)
offspring: 8 golden, 5 black, 2 brown

3.) You cross true-breeding red-flowered plants with true-breeding white-flowered plants, and the F1 are all red-flowered plants. The F2, however, occur in a ratio of 9 red: 6 pale purple: 1 white.

How many genes are involved in the inheritance of this color character?

Explain why the F1 are all red and how the 9:6:1 ratio of the phenotypes in the F2 occurred.

Solution Preview

Gene Ideas
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1.) Fur color in rabbits is determined by a single gene locus for which there are four alleles. Four phenotypes are possible: black, Chinchilla (gray color causes by white hairs with black tips), Himalayan (white with black patches on extremities), and white. The black allele (C) is dominant over all other alleles, the Chinchilla allele (C^ch) is dominant over Himalayan (C^h), and the white allele (c) is recessive to all others.

a.) A black rabbit is crossed with a Himalayan, and F1 consists of a ratio of 2 black to 2 Chinchilla. Can you determine the genotypes of the parents?

To summarize, there are 4 alleles: C, Cch, Ch, and c. They are dominant to each other in that order.

The parents in this problem are black and Himalayan, which means that their genotypes are C_ and Ch_ . Set up a Punnett square for this cross:

C ?
Ch CCh Ch?
? C? ??

Currently, there are two black offspring (CCh and C?), one that is black, Himalayan or Chinchilla (Ch?: it will be black if the missing allele is C, Chinchilla if the mystery allele is Cch, and Himalayan if it is Ch or c).

Because the F1 generation consists of only black and Chinchilla offspring, the Ch? offspring must be Chinchilla, which means that the missing allele from the C? parent is Cch. Adding that information to the Punnett square, we have:

C Cch
Ch CCh ChCch
? C? Cch?

Now, we have 2 black offspring (CCh and C?), one Chinchilla (ChCch), and one that can be either black or Chinchilla (Cch?). We know that this last one has to be Chinchilla, in order to get the correct ratio for the F1 generation. However, this could happen if the missing allele is Ch, or ...

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