Chapter 2, Problem 46P

Alleles of the IGF-1 gene in dogs, encoding insulin-like growth factor, largely determine whether a domestic dog will be large or small. Dogs with an ancestral domi-nant allele are large, whereas dogs homozygous for the mutant recessive allele are small. Chondrodysplasia, a short-legged phenotype (as in dachshunds and basset hounds), is caused by a dominant gain-of-function allele of the FGF4 gene. The MSTN gene encodes myostatin, a regulator of muscle development. Dogs with a dominant ancestral allele of the MTSN gene have normal muscle development, while dogs homozygous for recessive mutants in the MTSN gene are “double muscled” and have trouble running quickly. However, dogs heterozy-gous for the mutant allele run faster than either of the homozygotes. You breed a pure-breeding small basset hound ofnormal musculature with a pure-breeding “bully” whip-pet, a double-muscled large dog with normal legs.

a. What are the genotypes and phenotypes of the $F_1$ puppies?

b. If the $F_1$ of this cross is interbred, what proportion of the $F_2$ are expected to be fast runners and what propor-tion normal-speed runners?