Equine coat color

Horses exhibit a diverse array of coat colors and distinctive markings. A specialized vocabulary has evolved to describe them.
While most horses remain the same coat color throughout life, some undergo gradual color changes as they age. Most white markings are present at birth, and the underlying skin color of a healthy horse does not change. Certain coat colors are also associated with specific breeds, such as the Friesian, which is almost exclusively black.
The basic outline of equine coat color genetics has largely been resolved, and DNA tests to determine the likelihood that a horse will have offspring of a given color have been developed for some colors. Discussion, research, and even controversy continue about some of the details, particularly those surrounding spotting patterns, color sub-shades such as "sooty" or "flaxen", and markings.
The two basic pigment colors of horse hairs are pheomelanin ("red"), which produces a reddish brown color, and eumelanin, which produces black. These two hair pigment genes create two base colors: chestnut, which is fully red, and black, which is fully black.
All other coat colors are created by additional genes that modify these two base colors. The most common modifier creates point coloration of both red and black hairs, known as bay, which is classified as a base color as well. The vast range of all other coat colors are created by additional genes' action upon one of these three base colors.
In the absence of DNA testing, chestnut and bay can be distinguished from each other by looking at the mane, tail and legs for the presence of black points. There is a proposed allele that darkens a bay coat to seal brown, and the sooty gene is linked to other forms of dark bay.
Genetically, a chestnut horse is a horse without the ability to produce black pigment, while a black horse does not have dominant agouti to restrict their black pigment to points. The MC1R (extension) either binds alpha-MSH and signals for black and red pigment to be produced ('E' at extension), or it only signals for red ('e' at extension). ASIP (agouti) either blocks MC1R from binding to alpha-MSH and signalling for black ('A' at agouti), or it does not ('a' at agouti). The extension gene determines whether the cells can decide to produce black and red, and can be either E (able to produce black and red) or e (only able to produce red, as in chestnut). To be chestnut, a horse must have two copies of e, so the genotype is e/e. A horse with a genotype of E/E or E/e can still make black and red pigments and will be bay or black. Meanwhile, the agouti gene determines whether the cells can stop producing black. The A version of agouti means that it can, so as long as has E at extension the base color will be bay. The a version of agouti means the cells cannot stop producing black, so a horse with two copies of a (genotype a/a) and E at extension will be black rather than bay.
The word "points" is given to the mane, tail, lower legs, and ear rims with respect to horse coloration. The overall name given to a horse's coat color depends on the color of both the points and the body. For example, bay horses have a reddish-brown body with black points.
Point coloration is most often produced by the action of the agouti gene. It acts on the extension gene, when present, to suppress black color to all but the extremities of the horse; the legs, mane, tail and tips of the ears. If the extension gene is not present, the effect of agouti is not visible, as there is no black color present to suppress.
Other genes, such as those for white markings, may affect a horse's coat color in addition to agouti, and if present, can further alter or suppress black hair color and may mask any point coloration. In particular, Gray horses are born dark and lighten with age; if born bay, they will eventually lose point coloration as the body hair silvers with age, though often the points are the slowest areas of the body to go gray.
Point coloration may also be visible on horses with other dilution genes that act upon a bay base coat. These include:
A dilution gene that produces what looks like point coloration, but from a completely different genetic mechanism is the dominant Dun gene, which dilutes the color of the body coat but not the points, including primitive markings—a dorsal stripe down the back and, less often, horizontal striping on the upper legs. On a bay base coat, the dun gene leaves black points, producing a Bay Dun or "Zebra" Dun. But the gene also leaves the points dark when it appears with other base colors. These include the “blue dun” or grullo, which has a black base coat, and the red dun, which has a chestnut base coat.
Similarly, darker coloration at the points is also preserved in horses with the roan gene, a patterning gene, producing a body coat of mingled white and dark hairs, but leaving the points the darker base color in all horses, not just those carrying agouti.
Most other genes that produce spotting patterns or white markings allow point coloration produced by agouti to show except where masked by white depigmentation. There are not always separate names for a pattern over a bay base coat, but one exception is the Bay pinto, sometimes called ”tricoloured”.
A gray horse can be born any color, but as it gets older, some hairs turn white. Most will eventually develop a complete white or a "fleabitten" coat, which retains speckles of the horse's original color. Grays are sometimes confused with certain roan, dun, or white coat colors. In particular, most "white" horses are actually grays with a fully white hair coat. A gray horse is usually distinguishable from a dominant white or a cremello horse by dark skin, particularly noticeable around the eyes, muzzle, flanks, and other areas of thin or no hair. A roan has intermixed light and dark hairs similar to a young gray horse, but unlike a gray does not lighten to white. Dun horses have a solid-colored hair coat that also does not lighten with age. Gray horses are prone to equine melanoma.
Variations of gray that a horse may exhibit over its lifetime include:
Several different genetic allelic families produce colors that are lighter versions of the base colors, caused by dilution genes.
Cream dilution is an incomplete dominant gene that produces a lightened or "partial dilute" coat color when one copy of the allele is present and a fully dilute (or "double dilute") with two copies. The double cream dilute phenotypes overlap regardless of base coat color and often cannot be distinguished visually. Sometimes the creme allele is combined with an unrelated dilution gene from another family, which creates a cremello-like coat. Such coloration is called a "pseudo-double dilute." These distinctions usually require DNA testing to verify which alleles are present.
Mixtures of dliution genes produce colors such as "dunalino" — a red dun that also carries a single cream gene and thus has a pale gold coat, white mane and tail, and very faint primitive markings.
These patterns all have white hairs and often pink skin, varying from a fully white horse through the pinto patterns and smaller white markings to roan which only adds a few white hairs spread throughout the coat. These patterns can occur on top of any other color. The base color determines the color of the colored hairs, while the white patterns determine where and how many white hairs are present. Biologically, the white comes from a lack of pigment cells. Many different genetic alleles create these patterns.
There are a group of coat patterns caused by the leopard gene complex. Not every horse with leopard genetics will exhibit hair coat spotting. However, even solid individuals will exhibit secondary characteristics such as vertically striped hooves and mottled skin around the eyes, lips, and genitalia, plus a white sclera of the eye. Several breeds of horse can boast leopard-spotted (a term used collectively for all patterns) individuals including the Knabstrupper, Noriker, and the Appaloosa. There are several distinct leopard patterns:
A pinto has large patches of white over any other underlying coat color. Sometimes called "Paint" in the western United States, a word that which technically refers to the American Paint Horse, a specific breed of mostly pinto horses with known Quarter Horse and/or Thoroughbred bloodlines. Other regional terms for certain pinto spotting patterns include "blagdon" in the UK. Pinto spotting is produced by a large number of genetic mechanisms, with dozens now mapped and identifiable through DNA testing.
Variations of pinto based on the observable color include:
Terminology variations based on the observable shape of the white patterns include:
Roaning adds white hairs to any of the other colors and, unlike gray, the color does not steadily lighten over the horse's lifetime, though there may be some minor color variation from year to year or especially between summer and winter coats.
Rabicano: A roan-style effect that is caused by a yet-to-be-mapped genetic modifier that creates a mealy, splotchy, or roaning pattern on only part of the body, usually limited to the underside, flanks, legs, tail and head areas. Unlike a true roan, much of the body will not have white hairs intermingled with solid ones, nor are the legs or head significantly darker than the rest of the horse.
One of the rarest colors, a true genetic white horse has white hair and fully or largely unpigmented (pink) skin. These horses are born white or mostly white and remain white for life. The vast majority of so-called "white" horses are actually grays with a fully white hair coat. A truly white horse occurs one of two ways: either by inheriting one copy of a dominant white ("W") allele that produces white when heterozygous but may be a genetic lethal if homozygous, or by inheriting two copies of a non-lethal dominant white ("W") allele that produces a white coat when homozygous. There are also some genetic lethal genes unrelated to the W allelic series: a foal homozygous for the frame overo gene will have a condition known as lethal white syndrome dies shortly after birth.
There are no "albinos" in the horse world. Albinos, defined as animals with a white coat with pink skin and reddish eyes, are created by genetic mechanisms that do not exist in horses. In some cases, homozygous dominant white (W) is thought to be an embryonic lethal, though this does not occur with all W alleles.
White markings are present at birth and unique to each horse, making them useful in identifying individual animals. Markings usually have pink skin underneath them, though some faint markings may not, and white hairs may extend past the area of underlying pink skin. Though markings that overlie dark skin may appear to change, the underlying skin color and hair growing from pink skin will not. The distinction when white markings confined to the face and legs or a few small body spots become extensive enough to constitute a white spotting pattern is usually determined by breed standards set by registries. White markings generally are now hypothesized to be a minimal expression of certain genes in the dominant white (W) allelic series.
Most horses have brown eyes with minor shade variations. Blue eyes are linked to the splashed white spotting allele, and cream dilution may produce a bluish-green eye color. The champagne and pearl genes also produce lightened eye colors in the blue or green shades. The leopard complex produces a white sclera around an otherwise dark eye.
The yellow or amber Tiger eye gene has been found only in the Puerto Rican Paso Fino and has two variants, Tiger-eye 1 (TE1) and Tiger-eye 2 (TE2), which are both recessive. There is no obvious link between eye shade and coat color, making this the first studied gene in horses to affect eye color but not coat color.
Exterior hoof wall color is usually linked to coat color. Most horses have a dark grayish hoof wall unless they have white leg markings, in which case they will have pale-colored hooves. The leopard complex gene will create a light and dark striped hoof, and many chestnut horses have brownish hooves that are somewhat lighter than the usual dark gray.
Registries have opened that accept horses (and sometimes ponies and mules) of almost any breed or type, with color either the only requirement for registration or the primary criterion. These are called "color breeds". Unlike "true" horse breeds, there are few, if any, unique physical characteristics required, nor is the stud book limited to only certain breeds or offspring of previously registered horses. As a general rule, offspring without the stated color are usually not eligible for recording with the color breed registry, although there are exceptions. The best-known color breed registries are for buckskins, palominos, and pintos.
Some horse breeds may have a desired coat color that usually breeds on as a characteristic that is part of the breed standard, in addition to distinctive physical characteristics and a limited stud book. They are not color breeds, and include the Friesian horse (must be uniformly black for mainstream registration), the Appaloosa (with Leopard complex patterns) and the American Paint Horse. In some of these breeds, though not all, offspring of animals registered in these stud books may be registered even if they do not have the desired color, sometimes with restrictions.