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 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 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 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.

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

Key Concepts

Alleles and Dominance

Alleles are different forms of a gene that can exist at a specific locus on a chromosome. In genetics, dominance refers to the relationship between alleles, where a dominant allele can mask the expression of a recessive allele. In the context of the IGF-1 and MSTN genes, the presence of dominant alleles determines traits such as size and muscle development in dogs.

Genotype and Phenotype

The genotype is the genetic constitution of an organism, while the phenotype is the observable physical or biochemical characteristics resulting from the genotype and environmental influences. In the breeding scenario described, the genotypes of the parent dogs will influence the phenotypes of their offspring, determining traits like size and muscle development.

Punnett Squares and Genetic Ratios

Punnett squares are a tool used in genetics to predict the genotypes of offspring from a cross between two parents. By analyzing the combinations of alleles from the parents, one can determine the expected proportions of different phenotypes in the offspring. This method is essential for calculating the expected proportions of fast and normal-speed runners in the F₂ generation from the interbreeding of the F₁ generation.