Further Discusson on Bay, Black & Chestnut
by C. Williams

It can take a little study to understand how the Agouti and Extension loci work together to create the three basic horse colors. This can be further complicated by be fact that geneticists believe there are more than two alleles at each locus.

Wild-Type Bay

Geneticists such as Sponenberg, contend that a third allele is possible at the Agouti locus. It is believed to be the original "wild-type" coloring that was present in the horse prior to domestication. It still occurs within the horse, but the allele is infrequent to non-existent in many breeds.

The Wild-Type allele A⁺ expresses itself in that the points (particularly the lower legs) tend to have less black than regular bay color. The black may extend only up to around the pasterns, and possibly onto the cannon, but portions of the cannon and lower leg are mixed with red pigmentation rather than being mostly solid black.

In other mammals, it has been observed that more dominant alleles tend to result in more red coloring and less black. If this holds true for horses, then we would expect that the A⁺ allele is dominant to the A allele (which in turn is dominant over the a allele.)

While this doesn't really amount to a new color classification... it does result in two fairly known and easy-to-distinguish subtypes within basic bay.

Anyone who has observed horses, knows that bays come in more shades than just two. Undoubtedly, there are either more alleles at the A locus, or there are other modifying loci that cause a wide range of shades from dark to light, redder or browner among horses. At the present time, specific loci or alleles that determine all possible shades of bay color have not been established. This is likewise true for chestnut and black. (In the case of black, the visible differences are much less noticeable).

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Dominant Black

It has been postulated that a Dominant Black allele can occur at the Extension locus, and is usually given the designation as E^D.

Some evidence to this is suggested within the Arabian breed, but it does not seem to be a factor in most breeds.

It is thought that the E^D allele at the Extension locus will cause a horse to be black, instead of red, and overrides (or perhaps seems immune to) whatever alleles are present at the Agouti locus that cause bay color.

For further reading and discussion on this topic, please refer to the book "Equine Color Genetics" by D. Phillip Sponenberg.

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Views expressed herein are those of the writers and compilers of the various information. Reference sources are cited where applicable. Copyrights are the property of the respective authors.