This post has been revised with new photographs here.
I got a new book this week: Equine Color Genetics third edition, by Philllip Spoenenberg. I have the first two editions–and how things have changed since the first edition came out! Even the second edition had only four types of dilution genes. Now there are six, with at least one more that has not been located yet.
Lineback duns and creams were clearly separate by the second edition, which also greatly expanded on silver dapple and added champagne. But the third edition added pearl, mushroom and a rare dilution, probably recessive, found in Arabians.
Before starting to look at the effects of the dilution genes, not to mention the other genes that affect horse color, it is important to realize that horses, like most mammals, have two kinds of pigment. One, eumelanin, is black, and while some of the dilution genes may affect it, the kind of brown that produces the chocolate Labrador is not known to occur in horses. The other pigment, called phaeomelanin, varies from rich red-brown to a lighter golden red. We’ll call that red, but that color can also be changed by other genes.
The three basic horse colors are chestnut, bay, and black. (Seal brown may be a fourth color genetically, but that is still under investigation.) Patterns of white, interspersed white hairs, or dilution may act on any of these colors, as may a general scattering of black hairs through the coat. But these three colors are the base for all horse colors. DNA tests are now available for the genes that produce all of these colors.
Chestnut is predominantly red, including mane, tail and lower legs. The mane and tail may be lighter than the body (often called flaxen, and sometimes with interspersed white hairs) or darker than the body (usually due to interspersed black hairs.) The dark shades of chestnut, called liver chestnut, often have interspersed black hairs over the entire body. Chestnut is due to a recessive form of the same gene, called extension, that produces yellow Labrador Retrievers. Chestnut is recessive to normal extension (which allows black mane and tail) but in contrast to dogs, black can occur in the coat. Recessive means that chestnut to chestnut breedings can produce only chestnut foals, but bay to bay (or bay to black or black to black) can produce chestnut.
Bay horses have red on the body, but the mane, tail and lower legs are black. Interspersed black hairs are again a possibility. In addition, many bay horses have some body hairs (most numerous on the upper part of the horse) which have red bases but black tips. This type of hair, with a band of red on a hair with black tips (and sometimes even black bases) is very common in mammals, and is called agouti. Bay is in fact an agouti gene. and is dominant to non-agouti.
Black is most commonly due to non-agouti. Black horses have primarily black hair. There is a separate gene, called mealy, that can produce lighter shading on the muzzle, though there is some evidence that a similar effect can result from an agouti gene called tan-point. Black is usually recessive to bay–that is, two bay parents can have a black foal, but it would be very unusual (and probably an indicator of the rare dominant black) for two black parents to have a bay foal.
Any of these colors may have white markings, and as long as the markings are confined to face and lower lets, the horses will still be called chestnut, bay or black. A bay, for instance, can have four white stockings and still be a bay. Only the most extreme white markings can hide which base color is present.
I’ll be blogging on more of the horse color genes in the next few weeks. If you want a primer on basic genetics, check out my website on coat color genetics in dogs.