Q: - in dogs, the mexican hairless phenotype never breeds true. when mexican hairless are interbred the offspring are 2/3 hairless and 1/3 normal. it is also observed that some deformed puppies are born dead. propose a genetic explanation for these observatios.

Explanation:

The problem says that the hairless phenotype never breeds true. That means that it's not the result of a homozygous genotype (H₁H₁ or H₂H₂), so it is caused by the heterozygous genotype (H₁H₂).

The expected offspring from the cross between two Mexican hairless would be:

And the expected phenotypic ratio 3:1. However, the observed offspring shows a 2:1 ratio. What's happening?

If the observed phenotypic ratio in the offspring of a monohybrid cross (a single gene with two alleles) is 2:1, we can suspect that one of the genotypes is lethal in homozygosis and therefore does not appear in the progeny (the puppies are born dead).

If we proposed that the H₂ allele is lethal in homozygosis, then:

The phenotypic ratios change to 2:1, as observed in the experiment.

is this for the e2020 class