Gray horse

Gray is common in many breeds. Today, about one horse in 10 carries the mutation for graying with age.[2] The vast majority of Lipizzaners are gray, as are the majority of Andalusian horses. Many breeds of French draft horse such as the Percheron and Boulonnais are often gray as well. Gray is also found among Welsh Ponies, Thoroughbreds, and American Quarter Horses. All of these breeds have common ancestry in the Arabian horse. In particular, all gray Thoroughbreds descend from a horse named Alcock's Arabian, a gray born in 1700.[3] The gray coat color makes up about 3% of Thoroughbreds.[4]

Gray also affects spotting patterns of pintos and leopard complex horses such as Appaloosas. Its effects wash out the contrast of the markings of these patterns, sometimes colloquially described as "ghosting."

A gray mare with suckling foal. The mare has a white hair coat, but the underlying black skin still confirms that she is a gray and not white. The light hairs around the foal's muzzle and eyes indicate that it will gray like its mother. Not all foals show signs of graying this young.

A gray foal may be born any color. However, bay, chestnut, or black base colors are most often seen. As the horse matures, it "grays out" as 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. The changing patterns of white and dark hairs have many informal names, such as "rose gray," "salt and pepper," "iron gray", or "dapple gray." As the horse ages, the coat continues to lighten, often to a pure white. Some horses develop pigmented reddish-brown speckles on an otherwise white hair coat. Such horses are often called "fleabitten gray".

Different breeds, and individuals within each breed, take differing amounts of time to gray out. Thus, graying 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 means that the same gray horse will appear to be a different color over time, sometimes resulting in a need to change the color noted on breed registry papers.[5] Other times, people traveling with gray horses who have a pure white hair coat have encountered problems with non-horse-oriented officials such as police officers or border guards who are unclear about a horse who has papers saying it is "gray" when the horse in front of them appears white.[6]

To further complicate matters, the skin and eyes may be other colors if influenced by other factors such as white markings, certain white spotting patterns or dilution genes.

Gray color changes

Name	Image	Description
Foals		Horses in their first year begin to show a few white hairs as their foal coat sheds. Some foals show gray hairs around their eye, muzzle and other "soft" areas at birth, others do not show white hairs until they are almost yearlings.
Young gray horses		An intermediate stage typically seen in young horses in the early stages of turning gray is when white hairs are mixed with the dark birth color. In horses born black or dark bay, the horse shows mostly black and white hairs intermingled on the body. This is sometimes called "salt and pepper", "iron gray", or "steel gray." This is the most common intermediate form of gray, which can give a silvery look to the coat. A reddish tinge, called a "rose gray", describes this intermediate stage for a horse born a chestnut or bright bay. Young horses just starting to gray out are sometimes confused with roans, but a gray continues to lighten with age, while a roan does not. Roaning also causes fewer white hairs on the legs and head, giving the horse the appearance of dark points, which is usually not true of gray.
Dapple gray		Light and dark areas form a dappled pattern of dark rings with lighter hairs on the inside of the ring, scattered over the entire body of the animal. Not all grays dapple, and it is an intermediate stage, usually seen in young adult horses, often considered highly attractive. The horse will continue to become lighter. Dappled grays should not be confused with the slight dappling "bloom" seen on horses of other colors that are in excellent condition, as "bloom" dapples disappear should the horse lose condition.
Late stage graying		As grays become lighter with age, a few areas, especially around the flanks and legs, retain some color
Complete depigmentation		Nearly all hairs are white. A horse at this stage may be done changing color, or may begin to develop fleabitten pigment. Gray horses with a completely white coat can be distinguished from a white horse by their underlying black skin, particularly around the eyes, muzzle, and genital area.
Fleabitten gray		Flea-bitten gray is a color consisting of a white hair coat with small pigmented speckles or "freckles". The flea-bitten pattern is seen primarily in heterozygous Grays.[7] Most horses who become flea-bitten grays still go through a brief period when they are pure white. The amount of speckling varies between individuals and density of speckling may increase as the horse ages. 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.
Blood marks		One unique form of gray, genetically related to flea-bitten gray, are "blood marks" or a "bloody shouldered" horse. This is an animal that is so heavily pigmented on certain parts of the body, usually the shoulder area, that it appears as an irregular, almost solid pattern—as if blood had been spilled on the horse, hence the name. Blood marks can change size and shape as the horse ages. Arabian horse breeders claim the Bedouin people considered the "bloody shoulder" to be a prized trait in a war mare and much desired.[8]

The gray gene (G) is an autosomal dominant gene.[1] In simple terms, a horse which has even one copy of the gray allele, regardless of other coat color genes present, will always become gray. This also means that all gray horses must have at least one gray parent. If a gray horse is homozygous (GG), meaning that it has a gray allele from both parents, it will always produce gray offspring no matter the color genetics of the other parent. However, if a gray horse is heterozygous (Gg), meaning it inherits one copy of the recessive gene (g), that animal may produce offspring who are not gray, depending on the genetics of the other parent and Mendelian inheritance principles. Genetic testing is now possible to determine whether a horse is homozygous or heterozygous for gray.[9] The gray gene does not affect skin or eye color, so grays typically have dark skin and eyes, as opposed to the unpigmented pink skin of white horses.[1]

In 2008, researchers at Uppsala University in Sweden identified the genetic mutation that governs the graying process. The study revealed that all gray horses carry an identical mutation that can be traced back to a common ancestor that lived at least two thousand years ago.[2] 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.[7]

Gray is controlled by a single dominant allele of a gene that regulates specific kinds of stem cells. Homozygous grays turn white faster, are more likely to develop melanomas, and are less prone to develop the "fleabitten" speckling than heterozygous grays.[7]

Researchers suggest the pigmented speckles of the "fleabitten" gray, as well as more intense reddish-brown colored areas called "blood" markings, may be caused by a loss or inactivation of the gray allele in some of the somatic cells as that would explain why the speckles are more common on heterozygous grays than homozygotes.[7]

Melanoma in gray horses

Main article: Equine melanoma

Perianal gray horse melanoma

Perianal gray horse melanoma, histology of tumor

The identification of the gray mutation is of great interest in of medical research since this mutation also enhances the risk for melanoma in horses: some studies have suggested as many as 80% of grays over 15 years of age have some form of melanoma.[10] Growth rate depends on the type, and many are slow-growing, but over time, many develop into a malignant melanoma.[11]

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.[7] Some studies indicate as many as 66% of melanomas become malignant, though other studies have found much lower rates, and in one case, zero.[12]

Throughout history, both gray and white horses have been mythologized. As part of its legendary dimension, the gray horse in myth has been depicted with seven heads (Uchaishravas) or eight feet (Sleipnir), sometimes in groups or singly. There are also mythological tales of divinatory gray horses who prophesy or warn of danger.

• Albinism
• Equine coat color
• Equine coat color genetics

• "Introduction to Coat Color Genetics" from Veterinary Genetics Laboratory, School of Veterinary Medicine, University of California, Davis. Web Site accessed January 12, 2008

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