This post has been reissued and updated with new photographs here.
We have now looked at how the basic colors, black, seal brown, bay and chestnut, are produced by the interaction of the agouti locus (which determines where black and red pigment will be produced) and the extension locus, which determines whether black or red pigment can be produced, noting along the way that at the agouti locus more red is dominant to more black, while at the extension locus the opposite is the case.
We have also looked at a number of loci with alleles that modify the appearance of black and/or red pigment: dun, cream, champagne, silver dapple, mushroom and Arab dilute. Two types of dilution widespread in mammals do not seem to be present in horses: the type of brown seen in chocolate Labrador retrievers, and the type of “blue” (really a gray shade) seen in blue Great Danes and Dobernmans. In both cases, the colors are modifications of black pigment and can be found by genetic testing, but have not been found in horses. We do, however, have blue and gray horses, but the cause is quite different.
In addition to the base colors and their modifications, horses frequently have white hairs. Even the palest of cream horses will have pigment granules in the hairs. True white hairs, however, do not have pigment granules. The granules are placed in the growing hair by melanosomes, which originally form as part of the neural crest in the fetus, and then migrate to their final position in the skin. If they fail to reach some part of the skin, that part will be white, and usually the skin will lack pigment as well. Even the eyes are likely to be blue if surrounded by pink skin and white hair. White on the legs often is pared with light or striped hooves.
Stars, snips, and other white facial markings are very common in horses, as are white feet and ankles. The terminology for these markings varies, based on how extensive they are and exactly where they occur. These minor markings depend on a number of genes as well as chance–genetic clones may have different markings, suggesting that even the environment within the mare may have an effect. Generally chestnuts have more white markings than bays, which in turn have more than blacks, so the other color genes may affect melanocyte migration. It has even been demonstrated that bays with one chestnut gene at the extension locus have more white than those with two wild-type genes.
Aside from these minor markings and scars, white hair on horses falls into two categories. Grey, roan and some near-roan variants fall into a group in which individual white hairs are interspersed through the coat. In greys, this is progressive with age–greys are born colored and become progressively lighter with age. Roans are born with white hairs in the coat and stay the same or darken with age. A second group includes a number of types of white spotting: tobiano, frame, splash, dominant white, two types of sabino, manchado and the leopard complex.
Over the next few weeks I’ll be covering the genes that produce these white areas and interspersed hairs.