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.

What are the genotypes and phenotypes of the F₁ puppies?

Key Concepts

Alleles and Genotypes

Alleles are different versions of a gene that can exist at a specific locus on a chromosome. In this context, the dominant and recessive alleles of the IGF-1 and MSTN genes determine the size and muscle development of dogs. The genotype refers to the genetic makeup of an organism, which can be homozygous (two identical alleles) or heterozygous (two different alleles). Understanding these terms is crucial for predicting the traits of the F₁ puppies.

Dominant and Recessive Traits

Dominant traits are expressed in the phenotype even when only one copy of the allele is present, while recessive traits require two copies to be expressed. In the case of the IGF-1 gene, the dominant allele results in a large phenotype, whereas the recessive allele leads to a small phenotype. This concept is essential for determining the expected phenotypes of the offspring based on the parental genotypes.

Phenotypic Ratios in Offspring

Phenotypic ratios describe the expected distribution of different traits in the offspring resulting from a genetic cross. In this scenario, the cross between a small basset hound (homozygous recessive for size) and a large bully whippet (homozygous dominant for size) will yield F₁ puppies that are heterozygous for the IGF-1 gene, resulting in a uniform phenotype of large dogs. Understanding how to calculate these ratios is key to predicting the traits of the F₁ generation.