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When individuals with the genotype “AA” and “Aa” are phenotypically different, and the trait encoded by allele “A” is more weakly expressed in the heterozygous “Aa” individual than in the homozygous “AA” individual, then the allele “A” is designated as incompletely dominant. In this case the heterozygous individual has a phenotype that can be an intermediate phenotype (that is, an “average”) between individuals with the genotype “AA” and “aa.”

Now let us discuss genes that can amplify, weaken, or otherwise modify the action of other genes. Such genes are called genetic modifiers. An organism’s phenotype can be formed by the action of two or more of these non-allelic or complementary genes, which in combination create an effect other than what the genes would on their own. Examples in horses are the genes Extension and Agouti, the first of which codes for the production of black pigment, and the second of which distributes the black pigment throughout the horse’s body. Another genetic modifier is a gene that masks the expression of non-allelic genes, called epistatic.