Gray (horse)

Gray
	A gray Lipizzan horse. Grays are typically born a darker color, and their hair coat will be pure white before they are 10 years old as they age
Other names	Grey
Variants	dapple gray; flea-bitten gray; iron gray; rose gray;
Genotype
Base color	Any
Modifying genes	Graying gene (G) dominant when single allele present
Phenotype
Body	Born any color, progressively lightens with age until completely white, flea-bitten form may develop red hairs in a freckling pattern throughout coat, sometimes increasing with age
Head and Legs	May not gray at same rate as body, head may be first part of body to lighten, legs may be among last parts of body to lighten
Mane and tail	May not gray at same rate as body, lighter or darker
Skin	Usually black, except under white markings present at birth
Eyes	Usually dark brown, unless base color affected by other genes which lighten eye color
Other notes	Dominant over all other coat colors, when gray gene is present, horse will always become gray, may be masked if horse's base color is white

Last updated November 04, 2019

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.^[1] 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.

Equine coat color Horse coat colors and markings

Horses exhibit a diverse array of coat colors and distinctive markings. A specialized vocabulary has evolved to describe them.

The horse is one of two extant subspecies of Equus ferus. It is an odd-toed ungulate mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature, Eohippus, into the large, single-toed animal of today. Humans began domesticating horses around 4000 BC, and their domestication is believed to have been widespread by 3000 BC. Horses in the subspecies caballus are domesticated, although some domesticated populations live in the wild as feral horses. These feral populations are not true wild horses, as this term is used to describe horses that have never been domesticated, such as the endangered Przewalski's horse, a separate subspecies, and the only remaining true wild horse. There is an extensive, specialized vocabulary used to describe equine-related concepts, covering everything from anatomy to life stages, size, colors, markings, breeds, locomotion, and behavior.

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.

Arabian horse Horse breed originating in the Middle East

The Arabian or Arab horse is a breed of horse that originated on the Arabian Peninsula. With a distinctive head shape and high tail carriage, the Arabian is one of the most easily recognizable horse breeds in the world. It is also one of the oldest breeds, with archaeological evidence of horses in the Middle East that resemble modern Arabians dating back 4,500 years. Throughout history, Arabian horses have spread around the world by both war and trade, used to improve other breeds by adding speed, refinement, endurance, and strong bone. Today, Arabian bloodlines are found in almost every modern breed of riding horse.

The Thoroughbred is a horse breed best known for its use in horse racing. Although the word thoroughbred is sometimes used to refer to any breed of purebred horse, it technically refers only to the Thoroughbred breed. Thoroughbreds are considered "hot-blooded" horses that are known for their agility, speed, and spirit.

The American Quarter Horse, or Quarter Horse, is an American breed of horse that excels at sprinting short distances. Its name came from its ability to outdistance other horse breeds in races of a quarter mile or less; some have been clocked at speeds up to 55 mph (88.5 km/h). The American Quarter Horse is the most popular breed in the United States today, and the American Quarter Horse Association is the largest breed registry in the world, with almost 3 million living American Quarter Horses registered in 2014.

People who are unfamiliar with horses may refer to gray horses as "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 to discern a gray horse from a white horse. White horses usually have pink skin and sometimes even 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.

Markings on horses are usually distinctive white areas on an otherwise dark base coat color. Most horses have some markings, and they help to identify the horse as a unique individual. Markings are present at birth and do not change over the course of the horse's life. Most markings have pink skin underneath most of the white hairs, though a few faint markings may occasionally have white hair with no underlying pink skin. Markings may appear to change slightly when a horse grows or sheds its winter coat, however this difference is simply a factor of hair coat length; the underlying pattern does not change.

White horses are born white and stay white throughout their lives. White horses may have brown, blue, or hazel eyes. "True white" horses, especially those that carry one of the dominant white (W) genes, are rare. Most horses that are commonly referred to as "white" are actually "gray" horses whose hair coats are completely white and may be born of any color and gradually "gray" as time goes on and take on a white appearance.

Roan is a horse coat color pattern characterized by an even mixture of colored and white hairs on the body, while the head and "points"—lower legs, mane and tail—are mostly solid-colored. Horses with roan coats have white hairs evenly intermingled throughout any other color. The head, legs, mane and tail have fewer scattered white hairs or none at all. The roan pattern is dominantly-inherited, and is found in many horse breeds. While the specific mutation responsible for roan has not been exactly identified, a DNA test can determine zygosity for roan in several breeds. True roan is always present at birth, though it may be hard to see until after the foal coat sheds out. The coat may lighten or darken from winter to summer, but unlike the gray coat color, which also begins with intermixed white and colored hairs, roans do not become progressively lighter in color as they age. The silvering effect of mixed white and colored hairs can create coats that look bluish or pinkish.

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 allele,^[1] 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.

A breed registry, also known as a herdbook, studbook or register, in animal husbandry and the hobby of animal fancy, is an official list of animals within a specific breed whose parents are known. Animals are usually registered by their breeders while they are young. The terms studbook and register are also used to refer to lists of male animals "standing at stud", that is, those animals actively breeding, as opposed to every known specimen of that breed. Such registries usually issue certificates for each recorded animal, called a pedigree, pedigreed animal documentation, or most commonly, an animal's "papers". Registration papers may consist of a simple certificate or a listing of ancestors in the animal's background, sometimes with a chart showing the lineage.

Depigmentation is the lightening of the skin or loss of pigment. Depigmentation of the skin can be caused by a number of local and systemic conditions. The pigment loss can be partial or complete. It can be temporary or permanent.

An allele is a variant form of a given gene. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation. A notable example of this trait of color variation is Gregor Mendel's discovery that the white and purple flower colors in pea plants were the result of "pure line" traits which could be used as a control for future experiments. However, most alleles result in little or no observable phenotypic variation.

Prevalence

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]

The Andalusian, also known as the Pure Spanish Horse or PRE, is a horse breed from the Iberian Peninsula, where its ancestors have lived for thousands of years. The Andalusian has been recognized as a distinct breed since the 15th century, and its conformation has changed very little over the centuries. Throughout its history, it has been known for its prowess as a war horse, and was prized by the nobility. The breed was used as a tool of diplomacy by the Spanish government, and kings across Europe rode and owned Spanish horses. During the 19th century, warfare, disease and crossbreeding reduced herd numbers dramatically, and despite some recovery in the late 19th century, the trend continued into the early 20th century. Exports of Andalusians from Spain were restricted until the 1960s, but the breed has since spread throughout the world, despite their low population. In 2010, there were more than 185,000 registered Andalusians worldwide.

Percheron A breed of draft horse from France

The Percheron is a breed of draft horse that originated in the Huisne river valley in western France, part of the former Perche province from which the breed takes its name. Usually gray or black in color, Percherons are well muscled, and known for their intelligence and willingness to work. Although their exact origins are unknown, the ancestors of the breed were present in the valley by the 17th century. They were originally bred for use as war horses. Over time, they began to be used for pulling stagecoaches and later for agriculture and hauling heavy goods. In the late 18th and early 19th centuries, Arabian blood was added to the breed. Exports of Percherons from France to the United States and other countries rose exponentially in the late 19th century, and the first purely Percheron stud book was created in France in 1883.

The Boulonnais, also known as the "White Marble Horse", is a draft horse breed. It is known for its large but elegant appearance and is usually gray, although chestnut and black are also allowed by the French breed registry. Originally there were several sub-types, but they were crossbred until only one is seen today. The breed's origins trace to a period before the Crusades and, during the 17th century, Spanish Barb, Arabian, and Andalusian blood were added to create the modern type.

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 pinto horse has a coat color that consists of large patches of white and any other color. The distinction between "pinto" and "solid" can be tenuous, as so-called "solid" horses frequently have areas of white hair. Various cultures throughout history appear to have selectively bred for pinto patterns.

The Appaloosa is an American horse breed best known for its colorful spotted coat pattern. There is a wide range of body types within the breed, stemming from the influence of multiple breeds of horses throughout its history. Each horse's color pattern is genetically the result of various spotting patterns overlaid on top of one of several recognized base coat colors. The color pattern of the Appaloosa is of interest to those who study equine coat color genetics, as it and several other physical characteristics are linked to the leopard complex mutation (LP). Appaloosas are prone to develop equine recurrent uveitis and congenital stationary night blindness; the latter has been linked to the leopard complex.

A color breed refers to groupings of horses whose registration is based primarily on their coat color, regardless of the horse's actual breed or breed type.

Changes in the color of gray horses

A gray mare with suckling foal. The light hairs around this foal's muzzle and eyes indicate that it will gray like its mother. Not all foals show signs of graying this young. Stutemitfohlen.jpg — A gray mare with suckling foal. The light hairs around this 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, 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 ages, the coat continues to lighten to a pure white or fleabitten gray hair coat. Thus, the many variations of gray coloring in horses are 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. 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 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.^{[ citation needed ]} 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.^{[ citation needed ]}

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.

Young gray horses

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 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 a horse born a chestnut or lighter bay color. While these colors are "graying out," both red and white hairs are often mixed on the body. Thus rose gray horses have a slight pinkish tinge to their graying coat. These horses 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

"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.

The "flea-bitten" gray

A horse that has completely changed its base coat will either be pure white or "flea-bitten" gray. Flea-bitten gray is a color consisting of a white hair coat with small speckles or "freckles" of red-colored hair throughout. Most horses who become flea-bitten grays still go through a brief period when they are pure white.

The flea-bitten 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 flea-bitten 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. Arabian horse breeders hold the view that traditions of the desert Bedouin people considered the "bloody shoulder" to be a prized trait in a war mare and much desired.^[5]

The flea-bitten pattern comes about because of somatic loss of the duplication that causes graying with age, enabling normal pigmentation to be reestablished. Generally only occurs in heterozygous Grays.

The genetics of gray

The gray gene (G) is an autosomal dominant gene.^[1] In simple terms, a horse which has even one copy of the gray allele, even if it has a gene for another coloring, will always become gray. If a gray horse is homozygous (GG), meaning that it has a gray allele from both parents, it will always produce gray offspring. 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 what color gene an offspring inherits from its other parent). Conversely, a gray horse must have at least one gray parent. Genetic testing is now possible to determine whether a horse is homozygous or heterozygous for gray, or if it does not carry the gene at all.^[6] 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 also revealed that gray horses carry an identical mutation that can be traced back to a common ancestor that lived thousands of 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.^[8]

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 gray 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.^[7]

Horse coat colors sometimes confused with gray

White horses

Many people who are unfamiliar with horses refer to a gray horse as "white". However, most white horses have pink skin and some have blue eyes. A horse with dark skin and dark eyes under a white hair coat is gray. However, a gray horse with an underlying homozygous cream base coat color may be born with rosy-pink skin, blue eyes and near-white hair. In such cases, DNA testing may clarify the genetics of the horse.

Roan

Some grays in intermediate stages of graying may be confused with a roan or a rabicano. Some heavily fleabitten grays may also be confused with roans. However, roans are easily distinguishable from grays: roan consists of individual white hairs on a dark base coat, usually with the head and legs of the horse darker than the rest of the body. Rabicanos also have intermixed white hairs primarily on the body with a dark head. With gray horses, the head is often the first area to lighten, especially around the eyes and muzzle. Also, roans do not lighten with age, while grays always do.

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 created by the action of leopard complex within breeds such as the Appaloosa and are seldom seen elsewhere.

Diluted colors

The dilution genes that create dun, cream, pearl, silver dapple and champagne coloring may occasionally result in confusion with gray.

Some horses with a particular type of dun hair coat known as a "blue dun", grullo, or "mouse" dun 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. Horses that are a light cream color are also not grays. These are usually cremello, perlino or smoky cream horses, all colors produced by the action of the cream gene. However, if a gray parent passes on the gene, the hairs will turn white like any other gray. Another cream-colored dilution, the pearl gene or "barlink factor", may also create very light-coated horses. Similarly, the champagne gene can lighten coat color, often producing dappling or light colors that can be confused with gray.

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 that color and it will not lighten. However, again, if one parent passes on the gray gene, the gray gene will again be dominant.

Mythology

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.

Notes

1 2 3 4 Locke, MM; MCT Penedo; SJ Brickker; LV Millon; JD Murray (2002). "Linkage of the grey coat colour locus to microsatellites on horse chromosome 25". Animal Genetics. 33 (5): 329–337. doi:10.1046/j.1365-2052.2002.00885.x. PMID 12354140. The progressive loss of colour in the hair of grey horses is controlled by a dominantly inherited allele at the Grey locus (G^G). Foals are born any colour depending on the alleles present at other colour determining loci. The progressive loss of colour in the hair of grey horses is controlled by a dominantly inherited allele at the Grey locus (GG). Foals are born any colour depending on the alleles present at other colour determining loci. After birth, horses carrying the GG allele begin to show white hairs that are intermixed with their original hair colour. Although the rate at which horses will turn grey is variable, the amount of white hair increases with age until the coat is completely white at maturity. Pigmentation of the skin and eyes is not affected by GG. Dark skin distinguishes the grey phenotype from that of pink-skinned cremello and white horses.
1 2 Edited Press Release. "Genetics of the Gray Horse Unraveled." Article # 12468. The Horse, Online edition, August 7, 2008
↑ Swinburne, JE; A Hopkins; MM Binns (2002). "Assignment of the horse grey coat colour gene to ECA25 using whole genome scanning". Animal Genetics. 33 (5): 338–342. doi:10.1046/j.1365-2052.2002.00895.x. PMID 12354141. All grey Thoroughbred horses trace back to the Alcock’s Arabian (b1700).
↑ Willett, P. (1989). The Classic Racehorse. London: Stanley Paul. p. 24. ISBN 0-8131-1477-2.
↑ "Holly". www.babsonarabians.com. Retrieved 4 April 2018.
↑ "Gray - Horse Coat Color DNA Testing." Animal Genetics, Incorporated.'. web page accessed August 29, 2008
1 2 Rosengren Pielberg, G; Golovko, A; Sundström, E; et al. (August 2008). "A cis-acting regulatory mutation causes premature hair graying and susceptibility to melanoma in the horse". Nat. Genet. 40: 1004–9. doi:10.1038/ng.185. PMID 18641652.
↑ Pielberg, Gerli Rosengren; Anna Golovko; Elisabeth Sundström; Ino Curik; Johan Lennartsson; Monika H Seltenhammer; Thomas Druml; Matthew Binns; Carolyn Fitzsimmons; Gabriella Lindgren; Kaj Sandberg; Roswitha Baumung; Monika Vetterlein; Sara Strömberg; Manfred Grabherr; Claire Wade; Kerstin Lindblad-Toh; Fredrik Pontén; Carl-Henrik Heldin; Johann Sölkner; Leif Andersson (2008). "A cis-acting regulatory mutation causes premature hair graying and susceptibility to melanoma in the horse". Nature Genetics. 40 (8): 1004–1009. doi:10.1038/ng.185. PMID 18641652.

Related Research Articles

Roan is a coat color found in many animals, including horses, cattle, antelope and dogs. It is defined generally as an even mixture of white and pigmented hairs that do not "gray out" or fade as the animal ages. There are a variety of genetic conditions which produce the colors described as "roan" in various species.

Palomino is a genetic color in horses, consisting of a gold coat and white mane and tail, the degree of whiteness can vary from bright white to yellow. Genetically, the palomino color is created by a single allele of a dilution gene called the cream gene working on a "red" (chestnut) base coat. Palomino is created by a genetic mechanism of incomplete dominance, hence it is not considered true-breeding. However, most color breed registries that record palomino horses were founded before equine coat color genetics were understood as well as they are today, therefore the standard definition of a palomino is based on the visible coat color, not heritability nor the underlying presence of the dilution gene.

A dilution gene is any one of a number of genes that act to create a lighter coat color in living creatures. There are many examples of such genes:

Bay is a hair coat color of horses, characterized by a brown body color with a black mane, tail, ear edges, and lower legs. Bay is one of the most common coat colors in many horse breeds.

Cream gene gene responsible for a number of horse coat colors

The cream gene is responsible for a number of horse coat colors. Horses that have the cream gene in addition to a base coat color that is chestnut will become palomino if they are heterozygous, having one copy of the cream gene, or cremello, if they are homozygous. Similarly, horses with a bay base coat and the cream gene will be buckskin or perlino. A black base coat with the cream gene becomes the not-always-recognized smoky black or a smoky cream. Cream horses, even those with blue eyes, are not white horses. Dilution coloring is also not related to any of the white spotting patterns.

The champagne gene is a simple dominant allele responsible for a number of rare horse coat colors. The most distinctive traits of horses with the champagne gene are the hazel eyes and pinkish, freckled skin, which are bright blue and bright pink at birth, respectively. The coat color is also affected: any hairs that would have been red are gold, and any hairs that would have been black are chocolate brown. If a horse inherits the champagne gene from either or both parents, a coat that would otherwise be chestnut is instead gold champagne, with bay corresponding to amber champagne, seal brown to sable champagne, and black to classic champagne. A horse must have at least one champagne parent to inherit the champagne gene, for which there is now a DNA test.

Silver dapple gene also known as the "Z" gene, that dilutes the black base coat color in horses

The silver or silver dapple (Z) gene is a dilution gene that affects the black base coat color. It will typically dilute a black mane and tail to flaxen, and a black body to a shade of brown or chocolate. It is responsible for a group of coat colors in horses called "silver dapple" in the west, or "taffy" in Australia. The most common colors in this category are black silver and bay silver, referring to the respective underlying coat color.

Equine coat color genetics determine a horse's coat color. Many colors are possible, but all variations are produced by changes in only a few genes. Extension and agouti are particularly well-known genes with dramatic effects. Differences at the agouti gene determine whether a horse is bay or black, and a change to the extension gene can make a horse chestnut instead. Most domestic horses have a variant of the dun gene which saturates the coat with color so that they are bay, black, or chestnut instead of dun, grullo, or red dun. A mutation called cream is responsible for palomino, buckskin, and cremello horses. Pearl, champagne and silver dapple also lighten the coat, and sometimes the skin and eyes as well. Genes that affect the distribution of melanocytes create patterns of white such as in roan, pinto, leopard, white, and even white markings. Finally, the gray gene causes premature graying, slowly adding white hairs over the course of several years until the horse looks white. Some of these patterns have complex interactions.

The dun gene is a dilution gene that affects both red and black pigments in the coat color of a horse. The dun gene lightens most of the body while leaving the mane, tail, legs, and primitive markings the shade of the undiluted base coat color. A dun horse always has a dark dorsal stripe down the middle of its back, usually has a darker face and legs, and may have transverse striping across the shoulders or horizontal striping on the back of the forelegs. Body color depends on the underlying coat color genetics. A classic "bay dun" is a gray-gold or tan, characterized by a body color ranging from sandy yellow to reddish brown. Duns with a chestnut base may appear a light tan shade, and those with black base coloration are a steel gray. Manes, tails, primitive markings, and other dark areas are usually the shade of the undiluted base coat color. The dun gene may interact with all other coat color alleles.

Grullo Color of horses in the dun family

Grullo or grulla, also called blue dun, gray dun or mouse dun, is a color of horses in the dun family, characterized by tan-gray or mouse-colored hairs on the body, often with shoulder and dorsal stripes and black barring on the lower legs. In this coloration, each individual hair is mouse-colored, unlike a roan, which is composed of a mixture of dark and light hairs. The several shades of grulla are informally referred to with a variety of terms, including black dun, blue dun, slate grulla, silver grulla or light grulla, silver dun, or lobo dun. Silver grulla may also refer to a grullo horse with silver dapple, regardless of shade. In the Icelandic horse, the grulla color is called gray dun, in the Highland pony it is called mouse dun, and in the Norwegian Fjord horse, grå or gråblakk.

Chestnut is a hair coat color of horses consisting of a reddish-to-brown coat with a mane and tail the same or lighter in color than the coat. Chestnut is characterized by the absolute absence of true black hairs. It is one of the most common horse coat colors, seen in almost every breed of horse.

Varnish roan describes a horse with coloration similar to roan, but with some changes in color over the years, though not to the extreme of a gray. This type of roaning only occurs in conjunction with the Leopard complex. Varnish roans are born with a dark base coat color, usually with some spotting. As the horse ages, white hairs increase over most of the body, and many spotted markings blur or fade. The varnish roan pattern often appears to spread from the white of any original markings. This color pattern is best known in the Appaloosa breed of horse.

Black is a hair coat color of horses in which the entire hair coat is black. Black is a relatively uncommon coat color, and it is not uncommon to mistake dark chestnuts or bays for black.

Sabino is a group of white spotting patterns in horses that affect the skin and hair. A wide variety of irregular color patterns are accepted as sabino. In the strictest sense, "sabino" refers to the white patterns produced by the Sabino 1 (SB1) gene, for which there is a DNA test. However, other horse enthusiasts also refer to patterns that are visually similar to SB1 as "sabino", even if testing indicates the gene is not present. Use of the term to describe non-SB1 "sabino" patterns in breeds that apparently do not carry the gene is hotly debated by both researchers and horse breeders.

The leopard complex is a group of genetically related coat patterns in horses. These patterns range from progressive increases in interspersed white hair similar to graying or roan to distinctive, Dalmatian-like leopard spots on a white coat. Secondary characteristics associated with the leopard complex include a white sclera around the eye, striped hooves and mottled skin. The leopard complex gene is also linked to abnormalities in the eyes and vision. These patterns are most closely identified with the Appaloosa and Knabstrupper breeds, though it is due to a very ancient mutation and used to be widespread.

Seal brown is a hair coat color of horses characterized by a near-black body color; with black points, the mane, tail and legs; but also reddish or tan areas around the eyes, muzzle, behind the elbow and in front of the stifle. The term is not to be confused with "brown", which is used by some breed registries to refer to either a seal brown horse or to a dark bay without the additional characteristics of seal brown genetics.

Dominant white Horse coat color and its genetics

Dominant white or white spotting is a group of genetically related coat color conditions in the horse, best known for producing an all-white coat, but also able to produce various forms of white spotting and white markings, several of which are sometimes referred to as sabino.

References

"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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[locke2002-1] 1 2 3 4 Locke, MM; MCT Penedo; SJ Brickker; LV Millon; JD Murray (2002). "Linkage of the grey coat colour locus to microsatellites on horse chromosome 25". Animal Genetics. 33 (5): 329–337. doi:10.1046/j.1365-2052.2002.00885.x. PMID 12354140. The progressive loss of colour in the hair of grey horses is controlled by a dominantly inherited allele at the Grey locus (G^G). Foals are born any colour depending on the alleles present at other colour determining loci. The progressive loss of colour in the hair of grey horses is controlled by a dominantly inherited allele at the Grey locus (GG). Foals are born any colour depending on the alleles present at other colour determining loci. After birth, horses carrying the GG allele begin to show white hairs that are intermixed with their original hair colour. Although the rate at which horses will turn grey is variable, the amount of white hair increases with age until the coat is completely white at maturity. Pigmentation of the skin and eyes is not affected by GG. Dark skin distinguishes the grey phenotype from that of pink-skinned cremello and white horses.

[Unraveled-2] 1 2 Edited Press Release. "Genetics of the Gray Horse Unraveled." Article # 12468. The Horse, Online edition, August 7, 2008

[swinburne2002-3] Swinburne, JE; A Hopkins; MM Binns (2002). "Assignment of the horse grey coat colour gene to ECA25 using whole genome scanning". Animal Genetics. 33 (5): 338–342. doi:10.1046/j.1365-2052.2002.00895.x. PMID 12354141. All grey Thoroughbred horses trace back to the Alcock’s Arabian (b1700).

[willett-4] Willett, P. (1989). The Classic Racehorse. London: Stanley Paul. p. 24. ISBN 0-8131-1477-2.

[5] "Holly". www.babsonarabians.com. Retrieved 4 April 2018.

[6] "Gray - Horse Coat Color DNA Testing." Animal Genetics, Incorporated.'. web page accessed August 29, 2008

[Nature_Genetics-7] 1 2 Rosengren Pielberg, G; Golovko, A; Sundström, E; et al. (August 2008). "A cis-acting regulatory mutation causes premature hair graying and susceptibility to melanoma in the horse". Nat. Genet. 40: 1004–9. doi:10.1038/ng.185. PMID 18641652.

[pielberg-2008-8] Pielberg, Gerli Rosengren; Anna Golovko; Elisabeth Sundström; Ino Curik; Johan Lennartsson; Monika H Seltenhammer; Thomas Druml; Matthew Binns; Carolyn Fitzsimmons; Gabriella Lindgren; Kaj Sandberg; Roswitha Baumung; Monika Vetterlein; Sara Strömberg; Manfred Grabherr; Claire Wade; Kerstin Lindblad-Toh; Fredrik Pontén; Carl-Henrik Heldin; Johann Sölkner; Leif Andersson (2008). "A cis-acting regulatory mutation causes premature hair graying and susceptibility to melanoma in the horse". Nature Genetics. 40 (8): 1004–1009. doi:10.1038/ng.185. PMID 18641652.