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.

answer : haversian

[explanation: -(compact bone is organized as parallel columns, known as haversian systems, which run lengthwise down the axis of long bones. these columns are composed of lamellae, concentric rings of bone, surrounding a central channel, or haversian canal, that contains the nerves, blood vessels, and lymphatic system of the bone)]