Genetics
The various shades of bay may be genetically produced by multiple factors, but a simple explanation of bay genetics is that "red" color, seen in the chestnut horse, represented by the recessive "e" allele; and black color, represented by the dominant "E" allele, are the two most basic coat color genes. All other colors are produced by the action of additional alleles acting on these two base colors.
A bay horse carries both the Extension (E) allele and a suppression gene known as the agouti gene (A).The agouti gene, dominant over the black gene, limits or suppresses the black coloring to black points, allowing the underlying red coat color to come through. Unlike other types of "point" coloring, such as that seen in Siamese cats, the black points characteristic of bay coloring are not produced by a dilution or albinism gene.
Because the extension (E) gene and agouti (A) gene can be either heterozygous or homozygous, the extent to which a bay passes on its color varies widely from one horse to another depending on its genotype and that of its mate. Also, a chestnut may carry the Agouti gene, which will be "masked" or not manifest until the horse is bred to a horse with the E allele and produces offspring with both genes.
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