The following colors are recognized by the Jockey Club:
BAY: The entire coat of the horse may vary from a yellow-tan to a bright auburn. The mane, tail and lower portion of the legs are always black, unless white markings are present.
BLACK: The entire coat of the horse is black, including the muzzle, the flanks, the mane, tail and legs, unless white markings are present.
CHESTNUT: The entire coat of the horse may vary from a red-yellow to a golden-yellow. The mane, tail and legs are usually variations of the coat color, unless white markings are present.
DARK BAY/BROWN: The entire coat of the horse will vary from a brown, with areas of tan on the shoulders, head and flanks, to a dark brown, with tan areas seen only in the flanks and/or muzzle. The mane, tail and lower portion of the legs are always black, unless white markings are present.
GRAY/ROAN: The Jockey Club has combined these colors into one color category. This does not change the individual definitions of the colors for gray and roan and in no way impacts the two-coat color inheritance principle as stated in Rule 1(E).
GRAY: The majority of the coat of the horse is a mixture of black and white hairs. The mane, tail and legs may be either black or gray, unless white markings are present.
ROAN: The majority of the coat of the horse is a mixture of red and white hairs or brown and white hairs. The mane, tail and legs may be black, chestnut or roan, unless white markings are present.
PALOMINO: The entire coat of the horse is golden-yellow, unless white markings are present. The mane and tail are usually flaxen.
WHITE: The entire coat, including the mane, tail and legs, is predominantly white.
—Jockey Club Registry
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The master of color is the gene. The gene is found inside the cell on the chromosome, coiled material formed in arkan pairs, a chain provided by each parent with the allele a blind toss from dam and sire to foal. Genes, like many tyrants, are small but manifest in a multiplicity of forms. Allele pairs dictate the genotype, which, due to the vagaries of expression, may or may not correlate precisely to phenotype: black, brown, bay, dun, grullo, buckskin, chestnut/sorrel, red dun, palomino, silver dapple, cremello, which subdivide to reflect allelic combinations of jet and raven and summer black; or dark and light and seal browns; slate, lobo, olive, smutty, or silver grullos, and so on; also the white markings, which increase upon the infinite with roans, or the gray of age, or rabicano, frosty, paint, or tobiano; this is to say nothing of the effects of dappling, foal transition, seasonal change, & Etc.
Nature manipulates her colors—or color happens, insofar as the gene has no Mind to mind the gene—either as alleles occupy loci in homozygous and heterozygous pairs, or through the wily machinations of epistasis, where brute dominance shoulders its autocratic way through the old bloodlines, while recessives wait in genetic shadow, eyeing the dominant pairs and biding their time until, in tandem, the recessives in a surprise move—
No, perhaps it’s better to render genetics a descriptive but meaningless math as it concerns the hard colors, these colors being chestnut, black, and bay:
ee
EE or Ee
&
EEAA, EEAa, EeAA, or EeAa
But math won’t satisfy. Why do we always want the story? A dominant allele storms the House of Agouti and seizes half its resources, producing a bay horse, AA or Aa. Most recessive combatants will ultimately join forces with the house to produce the expected black EE or Ee, but sometimes a chestnut, ee, emerges victorious from the House of Extension, outmaneuvering the blacks and dominant bays of Agouti.
One would imagine that mastering the houses—Agouti, Extension, Dun, Silver Dapple, Champagne, and their meddling servants Pangare, Sooty, Shade, Flaxen, Brindle—would allow for the rational construction of color, including the dilutes that form from the hard, fundamental colors. But then there is white. White is less a color than a superimposition. It is a pigmentless pattern, a roan or gray intrusion upon all the hard colors and their various configurations. A white is the only horse without pigment, though even the white horse has dark eyes, WhW. White serves to mask color, though color lives forever in the genes. Therefore, a white horse—or what seems a white horse—is capable of great reproductive surprises.
Ultimately you may breed for color just as you may breed for conformation, speed, strength, & Etc, but the organism itself exerts no will to form. The natural dispersal of color is neither random nor intentional. Which is all to say that there may be tyrants with no ambition for power.