How can genes at different locations interact to affect an animal trait?

In animal breeding, genes at different locations (loci) interact to influence traits through several mechanisms, primarily categorized as epistatic or polygenic effects.

Epistatic Gene Interaction

Epistatic variation occurs when the joint effects of genes at two or more loci determine the expression of a trait. In this relationship, an epistatic gene masks or prevents the expression of another gene, known as the hypostatic gene.12

A classic example is albinism in mice: the gene responsible for melanin production is epistatic to the genes that determine melanin placement (such as agouti or black). If an animal is homozygous for the albino gene, it cannot produce pigment, meaning the agouti or black alleles cannot be expressed regardless of their presence. Because identifying and controlling these complex interactions is difficult, they are rarely used deliberately in breeding programs.21

Polygenic Inheritance

Many quantitative traits, such as milk yield or growth rates, are polygenic, meaning they are determined by many genes acting together. Each individual gene typically produces only a slight effect on the phenotype. These genes may produce cumulative effects, where each gene adds a small amount to the expression of a trait.34