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Like most animals, the horse has two pigments: black (eumelanin), and red/yellow (pheomelanin). The difference between the production of red alone, and red plus black together is controlled by two alternative forms (alleles) of the Extension gene. The dominant “E” allele determines the production of the black pigment in addition to red, whereas the recessive “e” allele (also called the Red Factor) permits the expression of red only. Consequently, e/e horses have only red pigmented hair (chestnut, sorrel), while horses heterozygous or homozygous for the dominant allele E (E/e or E/E) can produce both red and black pigment. In a horse carrying at least one E allele, the body distribution of black pigmented hair is further controlled by alleles of the Agouti gene. A horse carrying the dominant “A” allele (either A/a or A/A) will have the black hair restricted to the points (mane, tail, legs) and the base color will be bay. In contrast, the recessive allele “a” does not restrict the black hair, and a homozygote (a/a) will have the black pigmented hair distributed evenly over the entire body, and the resulting base will be black. The seal brown coat color is characterized by a dark/black body color, with a black mane, tail and legs, and reddish (tan) highlights of varying extent around the muzzle, eyes, on the flanks, belly, and inner thighs. It has been long believed that seal brown is caused by a third allele (At) of Agouti, but this has been the subject of debates and some controversy, in part due to the lack of DNA-based evidence. In our recent research study we discovered that a specific DNA change in the Agouti locus is indeed responsible for At (a scientific manuscript is currently being prepared for publication). The following article featuring our At test was published in the April 2010 issue of the Paint Horse Journal (www.painthorsejournal.com). This article is posted here with the kind permission of the Paint Horse Journal:(click here) Hierarchy of interactions between Agouti alleles: bay A > At > a (bay A is dominant over At and 'a'; At is dominant over 'a') Expected base colors of an E/e or E/E horse: A/A, A/At, A/a = bay At/At, At/a = seal brown a/a = black The 'a' and At alleles are caused by separate changes in Agouti, which do not overlap. Thus to obtain a comprehensive Agouti allelic status (when it cannot be deduced from the breeding records), one has to perform two DNA tests, both offered by PDSAz: 1. The standard/conventional Agouti test, specific for the recessive 'a'. Everything that is not 'a' is interpreted as 'A' (but can be either bay-A, or seal brown-At). 2. The At-specific test will differentiate between bay A and At (this test done alone cannot distinguish between bay ‘A’ and ‘a’). |
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Seal Brown (At/a) Quarter Horse *
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* Seal Brown Quarter Horse photograph courtesy of Lisa Etzel