The basic coat colors of horses already described can be modified by a large number of other genes. These genes may dilute the phaeomelanin or eumelanin pigments. They may make some of the hairs on the body white. They may organize white or black hairs as to where on the body they occur. They may cause areas of the body, face or legs to be white, usually underlain by pink skin. They may affect the mane and tail more than the body, or leave certain areas unchanged when the rest of the coat is lightened. Finally, there are genes that darken the apparent color by adding black hair to the coat.
Not all of the genetic basis for these colors are understood, but I will try to explain the ones we know something about.
The first group is dilution genes.
At one time, the assumption was that dilute was palomino. Talk about a gross oversimplification! There are at least six different dilution loci in horses, three of which were initially thought to be palomino. These are:
Cream (C), which probably has two alleles in addition to wild-type and is responsible for dark-skinned palomino, buckskin, smoky, cremillo, perlino, and pearl, among others.
Dun (Dn), which produces a dorsal stripe as well as dilution, and which sometimes leaves the head and lower legs dark. Dun, red dun and grullo are all dun colors. Dun on bay is sometimes called buckskin, but it is genetically a completely different color from cream on bay.
Champagne (Ch), a dilution gene which on a chestnut base produces what was once called pumpkin-skinned palomino, but which on other colors can only be called champagne.
Silver Dapple (Z), called by different names in different breeds and sometimes called taffy. The allele producing dilution affects black more than red, and mane and tail more than body.
Mushroom (Mu) is a rare color which at first sight looks like silver dalpple, but is quite different genetically.
Finally, there is a form of dilution in Arabians which appears to be genetic.
The second group involve interspersed white hairs.
Grey (G) is the commoner form especially in Thoroughbred and Arabian breeds, and produces white hairs showing first on the head and increasing with time. Aged greys are not infrequently pure white, but they normally retain black skin. In fact, black-skinned whites are really greys.
Classic Roan (Rn, tends to darken with age, and white hairs may not occur or be vary sparse in head, legs, mane and tail. Scars on roans are often of the base color, lacking white hair.
Other types of roan occur, but in most cases the genetics are not well understood
The third group is made up of spotting genes.
At one time, these were limited to face and body markings, two types of pinto, and Appaloosa. We now know of a bewildering variety.
Face and leg markings are widespread and appear to be quantitative in inheritance, but their inheritance is poorly understood. Even nomenclature varies.
Tobiano (To) is a type of vertically oriented spotting in which the head normally remains plain or conservatively marked but the legs are white.
Sabino is still a catch-all term for paints not known to be due to a specific gene, though sabino-1 is often treated as a locus. Minimal sabinos often have both blazes and high white on the legs. Roaning can be part of this pattern.
Frame (Fr) is a type of horizontally oriented white spotting. Minimal Frames generally have very wide blazes or bald faces but pigmented legs.
Splashed white (Spl) has white spreading up from below and often nearly all-white heads.
Manchado and Brindle are rare and not well understood.
White (Wh) produces white with pink skin and dark eyes, but this phenotype may also be produced as the white extreme of several of the spotting genes.
Leopard (Lp) is often called appaloosa in North America, but the gene is found worldwide in breeds from ponies to draft horses. This gene produces a wide variety of patterns, but at least one LpLp allele seems to be necessary for all of them.
Finally, there are at least two mechanisms that darken the coat.
The genetics of sooty and shading are still uncertain.
Note that a horse may have any combination of these genes. In the trilogy I’m working on now Roi has a horse, Buttermilk, who is a palomino classic roan with LpLp giving her white splashes over her hips: EeEe C+CCr RnRnRn+ LpLpLp+.
I have old posts on all of these (see index, at the top of the page) which I will be freshening up with new photos and re-issuing over the next few months.