Gray or grey is a coat color of horses characterized by progressive silvering of the colored hairs of the coat. Most gray horses have black skin and dark eyes; unlike many depigmentation genes, gray does not affect skin or eye color Their adult hair coat is white, dappled, or white intermingled with hairs of other colors. Gray horses may be born any base color, depending on other color genes present. White hairs begin to appear at or shortly after birth and become progressively lighter as the horse ages. Graying can occur at different rates--very quickly on one horse and very slowly on another.
Gray horses appear in many breeds, though the color is most commonly seen in breeds descended from Arabian ancestors. Some breeds that have large numbers of gray-colored horses include the Thoroughbred, the Arabian, the American Quarter Horse, the Percheron, the Andalusian, the Welsh pony, and the most famous of all gray horse breeds, the Lipizzaner.
Many people who are unfamiliar with horses call gray horses "white." However, a gray horse whose hair coat is completely "white" will still have black skin (except under markings that were white at birth) and dark eyes. This is how most people can tell a gray horse from a white horse. White horses usually have pink skin and frequently have blue eyes. Young horses with hair coats consisting of a mixture of colored and gray or white hairs are sometimes confused with roan. Some horses that carry dilution genes may also be confused with white or gray.
While gray is commonly called a coat color by breed registries, genetically it may be more correct to call it a depigmentation pattern. It is a dominant gene, and thus a horse needs only one copy of the gray allele, that is, heterozygous, to be gray in color. A homozygous gray horse, one carrying two gray alleles, will always produce gray foals.
Gray also occurs in spotted horses such as pintos or Appaloosas, but its effects wash out the contrast of the markings of these patterns. For this reason, some color breed registries refuse or cancel registration of gray horses.
A gray foal may be born any color. However, bay, chestnut, or black base colors are most often seen. As the horse matures, white hairs begin to replace the base or birth color. Usually white hairs are first seen by the muzzle, eyes and flanks, occasionally at birth, and usually by the age of one year. Over time, white hairs replace the birth color and the horse changes slowly to either a rose gray, salt and pepper (or iron gray), or dapple gray. As the horse gets older, the coat continues to lightens further to a pure white or fleabitten gray hair coat. Thus, the many variations of gray coloring in horses are simply intermediate steps that a young horse takes while graying out from a birth color to a hair coat that is completely "white."
Different breeds, and individuals within each breed, take differing amounts of time to gray out. Graying therefore cannot be used to approximate the age of a horse except in the broadest of terms: a very young horse will never have a white coat (unless it is a true white horse), while a horse in its teens usually is completely grayed out. One must also be careful not to confuse the small amount of gray hairs that may appear on some older horses in their late teens or twenties, which do not reflect the gray gene and never cause a complete graying of the horse.
This change in hair color can be confusing. Many new horse owners, not understanding the workings of the gray gene, are disappointed to discover that their dapple gray horse turns completely white a few years later! Other times, people traveling with gray horses who have a pure white hair coat have run into problems with non-horse-oriented officials such as police officers or border guards who are confused over a horse who has papers saying it is "gray" when the horse is front of them appears white!
An intermediate stage in young horses that are in the early stages of turning gray is sometimes called "salt and pepper," "iron gray" or "steel gray." This coloring occurs when white and black hairs are intermingled together on the body, usually seen in horses that are born black or dark bay. This is the most common intermediate form of gray, which can give a silvery look to the coat. "Rose gray" is a term used to describe this intermediate stage for horse born a chestnut or lighter bay color. While these colors are "graying out," both red and white hairs are often mixed together on the body. Thus rose gray horses have a slight pinkish tinge to their graying coat. These horses are sometimes confused with roan, but a gray continues to lighten with age, while a roan does not.
"Dapple gray" is an intermediate stage not seen on all grays, but often considered highly attractive. It consists of a dark hair coat with "dapples," which are dark rings with lighter hairs on the inside of the ring, scattered over the entire body of the animal. It is another possible intermediate step in the graying process of the horse. Dappled grays should not be confused with the slight dappling "bloom" seen on horses that are very healthy or slightly overweight, as "bloom" dapples disappear should the horse lose condition.
A horse that has completely changed its base coat will either be pure white or "flea-bitten" gray. Fleabitten gray is a color consisting of a white hair coat with small speckles or "freckles" of red-colored hair throughout. Most horses who become fleabitten grays still go through a brief period when they are pure white.
The fleabitten pattern, like freckles on a human, can also vary: Some horses may appear almost pure white, with only a few speckles observed on close examination. Others may have so many speckles that they are occasionally mistaken for a roan or even a type of sabino. One unique form of fleabitten gray is the "bloody shouldered" horse. This is an animal that is so heavily flea-bitten on certain parts of the body, usually the shoulder area, that it almost appears as if blood had been spilled on the horse, hence the name. In the traditions of the desert Bedouin people who bred the Arabian horse, the "bloody shoulder" was a prized trait in a war mare and much desired. In some cases, a "bloody shoulder" might in theory also be caused the sabino or rabicano gene acting in addition to a gray coat.
The genetic process that causes the fleabitten color pattern is not well-understood at present.
In 2008, researchers at Uppsala University in Sweden identified the genetic mutation that governs the graying process. The study also revealed that gray horses carry an identical mutation that can be traced back to a common ancestor that lived thousands of years ago. The discovery that gray can be linked to a single animal provides an example of how humans have "cherry-picked" attractive mutations in domestic animals.
The gray mutation is caused by a 4.6-kb duplication in intron 6 of STX17 (syntaxin-17) that constitutes a cis-acting regulatory mutation. The study utilized 800 gray horses from 8 different breeds, and the genetic mechanism involved has not been found in any non-grey horses.
The identification of the gray mutation is also of great interest in of medical research since this mutation also enhances the risk for melanoma in horses; About 75% of grey horses over 15 years of age have a benign form of melanoma that in some cases develops into a malignant melanoma. The study of gray genetics has pointed to a molecular pathway that may lead to tumour development. Both STX17 and the neighboring NR4A3 gene are overexpressed in melanomas from gray horses, and those carrying a loss-of-function mutation in ASIP (agouti signaling protein) had a higher incidence of melanoma, implying that increased melanocortin-1 receptor signaling promotes melanoma development in Gray horses.
The varnish roan is another unusual coloration, sometimes seen in Appaloosa horses, that, like gray, can change with age, but unlike gray, the horse does not become progressively lighter until it is pure white. Varnish roans are thought to be linked to a gene complex within the Appaloosa breed and are seldom seen elsewhere.
Some horses with a particular type of dun hair coat known as a "blue dun," grullo, or grulla, appear to be a solid gray. However, this color is caused by the dun gene acting on a black base coat, and horses who are dun have all hairs the same color; there is no intermingling of white and dark hairs. Also, dun horses do not get lighter as they age. This particular color is most commonly seen in the American Quarter Horse, and because Quarter Horses can also be born gray or roan, there is sometimes a bit of confusion amongst aficionados of the breed.
Horses who are a light cream color are also not grays. These are usually cremello or perlino horses, colors produced by action of the cream gene. However, if a gray parent passes on the gene, the gray gene will be dominant over cremello. Another cream-colored dilition, the pearl gene or "barlink factor," may also create very light-coated horses.
In spite of its name, the silver dapple gene has nothing to do with graying. It is a dilution gene that acts only on a black coat, diluting the coat to a dark brown and the mane to a flaxen shade. Horses that express the silver dapple gene (and do not have the gray gene) are born with the color and it will not lighten. However, again, if one parent passes on the gray gene, the gray gene will again be dominant. Similarly, the champagne gene can lighten coat color, often producing dappling or light colors that can be confused with gray.