What phenomenon occurs when one gene masks the expression of another gene?

The phenomenon where one gene masks or suppresses the expression of another gene is known as epistasis. In epistasis, the interaction between alleles at different loci affects the phenotype in a specific way, meaning that the presence of certain alleles can completely alter the expected expression of another gene's alleles. This can lead to a situation where a single trait's expression is determined not solely by the alleles present at one locus, but is modified or completely controlling the outcome by alleles at another locus.

This contrasts with incomplete dominance, where the heterozygous phenotype is a blend of the two homozygous phenotypes, and codominance, where both alleles are fully expressed in the phenotype. Polygenic inheritance refers to traits that are controlled by multiple genes, leading to a continuous range of phenotypes rather than distinct categories. Each of these concepts represents different genetic interactions that do not involve the masking relationship characteristic of epistasis.