Question

Alleles at the IGF-1 locus in dogs, encoding insulin-like growth factor, largely determine whether a domestic dog will be large or small. Dogs with an ancestral dominant 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 negative regulator of muscle development. Dogs with a dominant ancestral allele of the MSTN gene have normal muscle development, while dogs homozygous for recessive mutants in the MSTN gene are "double muscled" and have trouble running quickly. However, dogs heterozygous for the mutant allele run faster than either of the homozygotes. You breed a pure-breeding small basset hound of normal musculature with a pure-breeding "bully" whippet, a double-muscled large dog with normal legs.

A)what is the genotype of the F1 puppies? Let I and i designate IFG-1 alleles, F and f - FGF4 alleles, and M and m - MTSN alleles

Answer: Ii Ff Mm

B) What is the phenotype of the F1 puppies?

ANSWER: The puppies are large, have short legs, and run fast.

C) If the F1 of this cross is interbred, what proportion of the F2 are expected to be fast runners ?

answer: 1/2

D) If the F1 of this cross is interbred, what proportion of the F2 are expected to be normal-speed runners?

Part D is the section I am having trouble with. The answer is not 1/2. Please help thank you!