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The existence of another allele “A⁺” has been proposed, which is responsible for the “Wild” bay color. However, again as of writing, it has not been confirmed.

The Extension locus also has a third allele “e^a,” which is recessive and present only in Black Forest Horses. In the homozygous state it leads to the formation of chestnut color—that is, phenotypically it is analogous to allele “e.” This can introduce confusion with the analysis of the red factor “e,” because a chestnut horse homozygous for the allele “e^a” can be erroneously identified as homozygous for “E.” However, chestnut offspring will be produced upon crossing with another chestnut horse.

Once I was asked why in horses pigment is frequently located on the periphery of the body. Apparently, it can be explained by the lower body temperature in these areas. An illustrative example is the color pattern in Siamese cats, which is characterized by a dark nose, ears, paws, and tail. The reason for this distribution of pigment is a temperature-sensitive form of tyrosinase, which facilitates the synthesis of dark pigment in the sections of the body with a reduced temperature. In horses, the existence of such a form of tyrosinase and its effect on the color has not yet been proven. However, Bowling and Ruvinski (2000) suggested that the reason for the outlying distribution of pigment in horses is indeed a biochemical mechanism, connected with a reduced temperature on the periphery of the body.