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

The appearance of normal foxes in the progeny of two platinum foxes suggests a genetic pattern involving multiple alleles, incomplete dominance, or dosage effects, where a normal allele can override the platinum mutant allele, resulting in a normal phenotype.

Step-by-step explanation:

Explanation for Platinum Fox Progeny

The inability of breeders to develop a pure-breeding platinum strain of foxes, despite repeated mating of two platinum foxes, suggests a form of genetic inheritance that does not follow simple Mendelian dominant-recessive patterns. In the given example, matings between two platinum foxes result in both platinum and normal-coated offspring. This scenario hints at a situation similar to the multiple alleles system for coat color in rabbits, where different alleles of a gene contribute to a range of phenotypes due to dominance hierarchies or dosage effects of gene products.

Considering the example of rabbits, where a wild-type allele might provide a full dose of pigment and mutant alleles provide a lesser dose or none, it is plausible that platinum foxes could be heterozygous for a mutant allele that is not completely dominant. This might mean that when two heterozygous platinum foxes are crossed, some of the offspring inherit two normal alleles (homozygous), resulting in the normal phenotype, consistent with Mendel's law of segregation. The fact that normal foxes appear in the progeny indicates incomplete dominance or a dosage effect where a single normal allele is sufficient to override the platinum mutation.

Answer 2

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

Step-by-step 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.