Dark Bay Vs. Seal Brown
Bay horses, which have a black mane, tail, and legs with a mixture of red and black hairs on the body coat, have one of several genotypes at the Agouti locus: A/A, A/At, or A/a. Seal brown horses, which have primarily black coats in addition to black "points", with reddish or tan hairs around their muzzle, eyes, girth and flanks, have one of two different genotypes at the Agouti locus: At/At or At/a. Both coat colors exhibit a broad range of potential shades due to a variety of factors including the bleaching or fading of black hair, nutrition, and the presence of sooty or countershading factors.
Many black horses fade, sunbleach, or otherwise undergo a lightening of their coat color with exposure to sunlight and sweat. Horses which do not undergo such fading may be called jet black, raven black, non-fading black, or sheer black. Furthermore, the black areas of horses that are not uniformly black may be subject to the same fading effects. A non-fading dark bay might then be darker than a sunbleached seal brown. Mineral and vitamin deficiencies can also contribute to a lighter coat, similar to sunbleaching.
Black-pointed horses that are not uniformly black often exhibit a trait called sootiness. A sooty coat exhibits a mixing of black or darker hairs more concentrated on the dorsal aspect (top) of the animal, and less prevalent on the underparts. Sootiness is thought to be a form of countershading. Horses without any sootiness are termed "clear-coated". Sootiness can be minor or quite extensive, and often includes dappling. Dark bay horses are typically sooty. The difference between the top-down distribution of the sooty trait and the lighter soft areas of a seal brown can also be difficult to distinguish from one another. The team of French researchers who developed the DNA test for the recessive a allele also discussed the possibility that Extension might be dosage-dependent. They found a statistically significant tendency for lighter bays to be heterozygous for the dominant, wildtype Extension allele (E/e, also written E+/Ee) while darker bays were more often homozygous (E/E). The authors acknowledged that other factors could play a role, and that the claim needed to be studied on a greater scale. This type of dosage-dependent behavior was not observed with Agouti.
Read more about this topic: Seal Brown (horse)
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