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A recessive allele of the receptor is designated “e,” and in the homozygous state the horse can produce only a defective receptor. As a result the melanocyte-stimulating hormone cannot activate the production of black pigment and the cells switch to synthesize the red/yellow pigment pheomelanin.

Thus, genotype “EE” or “Ee” determines the presence of black hair, and genotype “ee” produces red. A DNA test is available to test for the “e” (red factor) allele. With the aid of this test it is possible to determine if there is a chance to obtain foals with red color—for example, from a black horse. The absence of the “e” allele in the genotype (EE) means that a horse is homozygous for the dominant allele; the presence of allele “e” in a single copy (Ee) indicates that the horse is heterozygous. A horse homozygous for the recessive “e” allele (ee) can only have a chestnut base color.

The dominant allele “A” of the gene Agouti codes for an Agouti-signaling protein (ASIP), which is an antagonist of the melanocyte-stimulating hormone, neutralizing its action in some areas of the body. In these areas there isn’t any black pigment production and the hair is red. In sections in which Agouti-signaling protein is not produced (the mane, tail, lower part of the legs, and ear rims), the pigment synthesis is switched to the black eumelanin and the hair in these locations is black. Thus, dominant allele “A” leads to the formation of the bay color. The recessive allele “a” of Agouti results in the lack of production of ASIP, and in a homozygous state (aa) does not suppress the synthesis of black eumelanin throughout the entire integument. Therefore an “aa” horse, which also has the genotype “EE” or “Ee,” will have a black base color. A phenotype effect of the Agouti gene appears only in the presence of allele “E,” and directs the synthesis of eumelanin in necessary quantities. The Agouti gene has no effect in an “ee” homozygous horse because there is only the red pigment produced.