The Extension (red factor) gene codes for the MC1R protein which produces the black pigment (eumelanin). The dominant allele E produces a black pigment in the coat and the recessive allele e produces a red pigment. A horse with two copies of the e allele will have a chestnut coat. A horse with one or two copies of the E allele will have a bay or black coat. The effect of the extension gene is to be combined with the agouti gene (HORSE-AG) to obtain the three basic coat colours.
