Every coat color a horse carries is the product of two pigments and a small number of genes that control where and how much of each pigment is made. The system is layered: base color is set first, dilution alleles lighten it, and pattern genes redistribute or suppress it. Understanding how each layer works makes it possible to predict offspring color and to recognize what a horse’s appearance does and does not reveal about its genotype.
The Two Pigments
Horse color begins with two pigment types, both forms of melanin. Eumelanin is black-brown; phaeomelanin is red-yellow. Every coat color is a variation on how much of each pigment is produced, where in the hair shaft it deposits, and whether it is diluted or redistributed by modifier genes. A horse that produces only eumelanin appears black or brown. A horse that produces only phaeomelanin appears chestnut or sorrel. Most coat colors result from the interaction of genes that regulate the balance between the two.
The Extension Locus (MC1R)
The Extension locus, located on the MC1R gene, determines which pigments are possible. It has two primary alleles: E (dominant) allows eumelanin production, and e (recessive) suppresses it.
A horse with at least one E allele can produce black pigment in the areas the Agouti locus and other modifiers direct it. A horse homozygous for e (ee) cannot produce eumelanin at all; it produces phaeomelanin throughout and is chestnut or sorrel regardless of any other color gene. This is why chestnut breeds true: two chestnuts can only produce chestnut offspring, because neither parent carries the E allele to pass.
A third allele, E+ (sometimes called wild-type), has been identified in some populations and may produce slightly different shading in heterozygotes, but its practical effect on phenotype is minor compared to E versus e.
The Agouti Locus (ASIP)
Given that a horse carries at least one E allele and can therefore make eumelanin, the Agouti locus controls where that black pigment is distributed. The dominant A allele restricts eumelanin to the points (mane, tail, lower legs, and ear tips) while allowing phaeomelanin in the body coat. The result is bay. The recessive a allele places no such restriction: eumelanin distributes uniformly, producing a black horse.
The Agouti locus has no visible effect in a chestnut (ee) horse, because there is no eumelanin for it to restrict. This is why a chestnut horse can carry A or a without any change in appearance, and why two bays (each carrying hidden a alleles) can produce a black foal.
Three functionally distinct Agouti alleles are recognized in horses: A+ (wild bay, restricting black to a smaller point area with reddish-brown body), A (standard bay), and a (black). Wild bay is most common in Fjords and a few other primitive breeds. In practical genetics for most breeds, bay versus black is the principal Agouti distinction.
Brown (At)
A fourth allele at the Agouti locus, At, produces the color called brown or seal brown. Brown horses carry At in combination with A or in homozygous AtAt configurations (nomenclature varies by source). Phenotypically, brown horses appear very dark, nearly black, with characteristic soft areas of lighter tan or mealy color at the muzzle, flanks, and inside the elbows and stifles. The lightened soft areas distinguish a true brown from a faded black. Brown is common in some Thoroughbred and Standardbred lines and is often confused with black by visual inspection alone.
Base Colors
With Extension and Agouti, three base colors account for nearly all horses:
Chestnut (sorrel): ee at Extension; no black pigment anywhere. Body, mane, and tail all phaeomelanin, ranging from pale golden to deep liver depending on modifier genes and individual variation. Sorrel is the Western-use term for lighter red chestnuts; genetically the same.
Bay: E_ (at least one E) and A_ (at least one A). Black points on a red-to-brown body. Ranges from light sandy bay through blood bay to dark mahogany bay, with shade influenced by modifier loci.
Black: E_ and aa. Uniform black throughout. True blacks may sun-fade to a brownish cast; this is environmental bleaching of the hair shaft, not a dilution gene.
Dilution Genes
Several genes independently reduce pigment intensity without eliminating it. They act on one or both pigment types and are largely additive when combined.
Cream (SLC45A2)
The Cream gene, carried on the SLC45A2 locus, is the most influential dilution in most Western breeds. A single copy (heterozygous) produces visibly different results depending on the base color it acts on.
On chestnut: one cream copy produces palomino (a golden body with a white or near-white mane and tail).
On bay: one cream copy produces buckskin (a yellow to tan body with black points retained).
On black: one cream copy produces smoky black, which is often visually indistinguishable from a non-dilute black and requires genetic testing to confirm.
Two cream copies (homozygous) have a much stronger effect:
On chestnut: double dilute produces cremello (a cream to near-white horse with blue eyes and pink skin).
On bay: double dilute produces perlino, similar in body tone to cremello but the points retain enough eumelanin to appear orange or buff rather than white.
On black: double dilute produces smoky cream, a cream horse with slightly darker points and blue eyes.
Cremello, perlino, and smoky cream are called double dilutes. They are homozygous for Cream and will always pass one Cream allele to every foal.
Dun (TBX3)
Dun is caused by a variant in the TBX3 gene that reduces pigment in the body coat while leaving the primitive markings (dorsal stripe, leg bars, shoulder stripe, and sometimes cobwebbing on the face) at full intensity. Unlike Cream, Dun acts on both eumelanin and phaeomelanin.
Dun on chestnut produces red dun: a diluted reddish body with red or darker primitive markings.
Dun on bay produces classic dun (sometimes called bay dun or zebra dun): a tan to yellow-gold body with black points and a black dorsal stripe.
Dun on black produces grullo (also written grulla): a smoky blue-grey body, black points, and strong primitive markings.
Dun is dominant; a single copy produces the phenotype. The dorsal stripe is the most consistent diagnostic feature: virtually every true dun has one, though a faint stripe can appear in some non-dun horses and should not be taken as proof of dun without genetic confirmation.
Silver (PMEL17)
The Silver dilution gene (on the PMEL17 locus) acts selectively on eumelanin and has no visible effect on phaeomelanin. It dilutes black pigment in the mane, tail, and sometimes the body coat, while leaving red pigment unaffected.
On black horses, Silver produces a chocolate body with a flaxen or silver mane and tail, a color sometimes called silver dapple or chocolate flaxen.
On bay horses, Silver dilutes the black points to a flaxen or taupe color while the bay body remains largely intact.
Silver has no visible effect on chestnut horses, because chestnut produces no eumelanin for Silver to act on.
Silver is associated with multiple congenital ocular anomalies (MCOA) syndrome in breeds where it occurs, particularly Rocky Mountain Horses and related gaited breeds. Horses heterozygous for Silver may show mild ocular changes; homozygous horses are at higher risk for significant structural eye abnormalities. Genetic testing before breeding Silver carriers is advisable.
Pearl (Pseudoalbino)
Pearl is a recessive allele at the same locus as Cream. A single Pearl copy has no visible effect. Two copies of Pearl produce a subtle apricot or peach dilution, most visible on chestnut horses. One copy of Pearl combined with one copy of Cream produces the same double-dilute appearance as two Cream copies: a pale cream horse with blue eyes. This Pearl-plus-Cream combination is the source of what is sometimes called “pseudo-double-dilute” palominos or buckskins in Andalusian and Lusitano breeding.
Champagne (SLC36A1)
Champagne is a dominant dilution carried on the SLC36A1 gene. It acts on both pigment types, producing a diluted coat and characteristic amber or hazel eyes that may appear nearly blue at birth and darken with age. Skin under the coat is mottled pink with darker speckling, most visible on the muzzle and around the eyes.
On chestnut, Champagne produces gold champagne: a pale gold body with a lighter mane and tail.
On bay, Champagne produces amber champagne: a tan to gold body with darker (but still diluted) points.
On black, Champagne produces classic champagne: a pale khaki or bronze coat.
Champagne horses are frequently confused with their Cream-dilute counterparts (gold champagne resembles palomino, amber champagne resembles buckskin), but the mottled skin and amber eyes distinguish them. Genetic testing separates the two definitively.
Mushroom
Mushroom is a recessive dilution documented primarily in Shetland Ponies. Two copies produce a sepia or taupe body with a lighter mane and tail on what would otherwise be a chestnut horse. Its genetic locus has not been confirmed by peer-reviewed research as of 2021, and testing is available only through select laboratories.
White-Producing and Pattern Genes
Beyond dilution, a separate set of genes suppresses or redistributes pigment by controlling where melanocytes (pigment-producing cells) migrate during fetal development. These genes produce white markings, patterns, and in some cases, horses that appear wholly white.
Gray (STX17)
Gray is caused by a dominant mutation in the STX17 gene. Gray horses are born a non-gray color and progressively lose pigment with each hair cycle, becoming lighter over several years until they appear white or dapple grey. The underlying skin remains pigmented and the eyes remain dark, which distinguishes a gray horse from a white one.
Gray horses retain their non-gray base color genetically; a gray horse may be genetically bay, chestnut, or black. That base color is invisible by the time the horse has fully grayed, but it can be detected by DNA testing and may be relevant when breeding for color in offspring.
Gray is strongly associated with elevated risk of melanoma in horses. Most gray horses develop some melanoma lesions by middle age, most commonly under the tail, around the anus, and on the face. The majority remain benign for years; a minority become malignant. Horses with two copies of Gray (homozygous) appear to gray more rapidly and may have a higher melanoma risk.
Sabino (SB1) and Related Patterns
Sabino refers to several genetically distinct mechanisms that produce white markings with roaned or jagged edges, often including high white on the legs and face, belly spots, and white into the body. The best-characterized is the SB1 allele on the KIT gene.
A single SB1 copy produces variable white markings, often bold face markings and high stockings. Two copies of SB1 produce a maximally white horse (called a sabino white) with only small patches of color remaining. SB1 homozygotes are not the same as dominant white horses and the locus is distinct.
Other sabino-like patterns exist (collectively called polygenic sabino) and are not explained by SB1 alone. They appear to have a complex inheritance and are common in Clydesdales, Shire horses, and many stock breeds.
Tobiano (KIT gene region)
Tobiano is a dominant pattern caused by an inversion in the KIT gene region. It produces white patches that cross the topline (the back and croup), with rounded edges, and the head is usually solid-colored. Leg markings are typically white. Dark color generally remains on the flanks.
Tobiano is highly penetrant: nearly every horse with the allele expresses the pattern. A horse homozygous for Tobiano (TOTO) passes the allele to 100% of offspring and can be confirmed by genetic testing, which is commercially available.
The combination of Tobiano and Sabino produces a pattern called Tovero, which can include blue eyes and pinto markings from both pattern types.
Overo (EDNRB gene, Frame Overo)
Frame overo is caused by a mutation in the EDNRB gene. It produces white patches on the sides of the neck and body with horizontal, jagged edges that do not cross the topline. The mane and tail are typically dark. Face markings are common and often irregular.
Frame overo is associated with a serious genetic risk: two copies of the frame allele (OO) cause Overo Lethal White Syndrome (OLWS). Foals homozygous for frame are born white and with a malformed colon (aganglionosis of the intestine); they are unable to pass meconium and die within 24 to 72 hours without surgical intervention, which is uniformly unsuccessful. For this reason, breeding two frame carriers is considered inadvisable. Genetic testing for frame is reliable and widely available.
A horse may carry one frame allele with minimal visible white markings; some frame carriers appear solid-colored. Testing before breeding is the only reliable method to identify carriers.
Splashed White (MITF, PAX3 genes)
Splashed white is a group of related mutations (SW1 through SW6 and additional variants) affecting the MITF or PAX3 genes. The pattern produces a horse that appears to have been dipped head-down in white paint: bold blaze, white chin, and high white stockings, often with blue eyes. The face and lower body are white; the topline and upper body retain color.
Splashed white horses commonly have one or both eyes blue and are frequently deaf, particularly when homozygous for a splashed white allele or when carrying multiple SW variants. The deafness results from the same melanocyte migration failure that produces the white patterning, because cochlear function depends on pigment cells.
Dominant White (KIT gene)
Multiple dominant white alleles exist at the KIT locus, each arising as a distinct mutation. They are designated W1, W2, W3, and so on; more than 30 variants have been identified. Each produces a white or extensively white horse when heterozygous. Most are heterozygous in all living carriers because the homozygous state is thought to be embryonic lethal for most W alleles (the embryo cannot survive without any KIT signaling).
Dominant white horses have pink skin and dark or blue eyes. They differ from grays in lacking progressive depigmentation: they are white from birth or near-birth. They differ from double-dilute cremello/perlino horses in having pink skin but typically dark eyes.
Appaloosa Pattern (TRPM1 gene)
Appaloosa patterning is governed primarily by the LP (Leopard complex) locus, a mutation in the TRPM1 gene. LP controls the presence of the appaloosa pattern complex, which includes mottled skin, striped hooves, visible white sclera (the ring of white around the iris), and one or more coat patterns: blanket, snowflake, leopard (spots on white), few spot, and varnish roan.
A horse with one LP copy may show a blanket or roan pattern. A horse with two LP copies (LPLP) shows maximal patterning, which can include the nearly-white few-spot leopard phenotype.
LP homozygosity is associated with Congenital Stationary Night Blindness (CSNB), an inability to see in low-light conditions. Horses with two copies of LP are night blind; heterozygotes are generally not affected. The condition does not progress, but it affects behavior in dim lighting and should be accounted for in management and training.
Pattern-affecting modifiers at the PATN1 locus and others influence how much white appears in horses carrying LP, which is why two LP-carrying horses with similar genotypes may differ considerably in the extent of their patterns.
Epistasis and Combined Genotypes
Multiple genes in the same horse interact, and the result is often not deducible by reading the loci separately. A horse that is ee (chestnut) and carries Silver shows no visible effect of the Silver gene, because Silver only acts on eumelanin. A horse that is gray will progressively obscure every other color and pattern gene it carries. A frame overo carrier that is also tobiano will show both patterns, often producing bold, irregular pinto coloring.
Champagne and Cream together produce a very pale horse; double Cream with Champagne creates an extremely pale coat. Dun does not interact with Cream or Champagne in a simple additive way at the phenotype level, and the combination can be difficult to read without genetic testing.
Visual phenotype alone is insufficient for confident color prediction in matings involving multiple loci. A smoky black horse looks black; a frame carrier may look solid; a chestnut Silver carrier shows nothing. Genetic testing resolves these hidden genotypes and is available commercially for all major color loci including Extension, Agouti, Cream, Dun, Silver, Champagne, Tobiano, Frame, Splashed White, Gray, and LP.
Genetic Testing
Color genetics testing is performed from hair root samples or blood. Results report each tested allele with high accuracy. Testing is most useful when the horse’s phenotype does not reveal its genotype, as in the cases of smoky black, frame carriers without white, chestnut Silver carriers, and homozygous Tobiano horses. Breeders working with color-sensitive programs or breeds where recessive lethals (OLWS, CSNB) are a concern should test before pairing.
Most major equine genetics laboratories offer single-locus tests and coat color panels. The University of California Davis Veterinary Genetics Laboratory and Texas A&M Veterinary Genetics Laboratory are among the established academic testing facilities in the United States.
What is the difference between roan and gray in horses?
Roan and gray are often confused but have different mechanisms and are genetically unrelated. Roan is caused by a dominant allele at the KIT locus (Rn) that produces an intermixing of white and colored hairs throughout the body from birth, with the horse retaining its base color at the head, lower legs, mane, and tail. The pattern does not change with age; a roan born with a roan pattern stays roan. Gray, by contrast, is progressive: gray horses are born fully pigmented in their base color and lose pigment incrementally with successive hair cycles until the coat appears white or dapple-gray, typically over several years. A true gray horse has pigmented skin and dark eyes; the white hair is the result of melanocyte depletion, not absent pigmentation from birth. Classic roan (Rn allele) is thought to be homozygous lethal in most cases, meaning most roans are heterozygous carriers; the evidence for embryonic lethality of RnRn has been debated in the literature but remains the working assumption for most breeding programs.
Why does coat color alone not reliably predict a foal’s color?
Because many critical alleles are recessive or co-dominant and produce no visible signal in the carrier. A chestnut horse may carry a hidden Agouti allele (A or a) that has no phenotypic effect on a chestnut but determines whether black-based offspring are bay or black. A smoky black horse is genetically identical in appearance to a true black but carries one Cream allele; breed a smoky black to a chestnut and roughly half the foals will be palomino or buckskin, a result not predicted by looking at either parent. A frame overo carrier may be solid-colored. A Tobiano horse may or may not be homozygous. Genetic testing reports the actual alleles present and eliminates the guesswork, particularly for dilution carriers, pinto pattern carriers, and recessive-lethal loci like OLWS.