Question

On a fox ranch in Wisconsin, a mutation arose that gave a "platinum" coat color. The platinum color proved very popular with buyers of fox coats, but the breeders could not develop a pure-breeding platinum strain. Every time two platinums were crossed, some normal foxes appeared in the progeny. For example, in repeated matings of the same pair of platinnums, a total of 82 platinums and 38 normals was produced. The result was typical of all other such matings. Which of the following best accounts for these results?

Answer 1

Answer:

The allele for platinum coat color is lethal in the homozygous condition.

Explanation:

The breeders could not develop a pure-breeding platinum strain, so the foxes with platinum coats must be heterozygous (Pp).

When two platinums are crossed (Pp x Pp), we would expect a 3:1 phenotypic ratio in the offspring, with the dominant trait appearing higher frequency. However, the observed ratio is 82:38 ≅ 2:1

The phenotypic ratio 2:1 appears typically when one of the alleles is lethal in the homozygous condition. In this case, given that the platinum coat color is dominant and cannot be purebred, we can hypothesize that the genotype PP is lethal and does not appear in the offspring.