Bay (horse)

Last updated
Bay
BayMare.jpg
A bay mare
VariantsBright reddish-brown (A) to dark shades influenced by sooty or Seal brown At, wildtype A+
Genotype
Base colorBlack (E)
Modifying genes agouti gene (A)
Descriptionreddish-brown body coat with black point coloration
Phenotype
Bodyreddish-brown
Head and LegsBlack
Mane and tailBlack
SkinBlack
EyesBrown
Other notesBlack ear edges

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

Contents

The black areas of a bay horse's hair coat are called "black points", and without them, a horse is not a bay horse. Black points may sometimes be covered by white markings; however such markings do not alter a horse's classification as "bay". Bay horses have dark skin — except under white markings, where the skin is pink. Genetically, bay occurs when a horse carries both the Agouti gene and a black base coat. The addition of other genes creates many additional coat colors. While the basic concepts behind bay coloring are fairly simple, the genes themselves and the mechanisms that cause shade variations within the bay family are quite complex and, at times, disputed. The genetics of dark shades of bay are still under study. A DNA test said to detect the seal brown (At) allele was developed, but subsequently pulled from the market. Sooty genetics also appear to darken some horses' bay coats, and that genetic mechanism is yet to be fully understood.

Color variations and terminology

The horses of the New South Wales Mounted Police show some of the typical variations in the bay color. RAS NSW Mounted Police muster.jpg
The horses of the New South Wales Mounted Police show some of the typical variations in the bay color.

Bay horses range in color from a light copper red, to a rich red blood bay (the best-known variety of bay horse) to a very dark red or brown called dark bay,mahogany bay,black-bay, or brown (occasionally "seal brown"). The dark, brown shades of bay are referred to in other languages by words meaning "black-and-tan." Dark bays/browns may be so dark as to have nearly black coats, with brownish-red hairs visible only under the eyes, around the muzzle, behind the elbow, and in front of the stifle. Dark bay should not be confused with "Liver" chestnut, which is also a very dark brown color, but a liver chestnut has a brown mane, tail and legs, and no black points.

The pigment in a bay horse's coat, regardless of shade, is rich and fully saturated. This makes bays particularly lustrous in the sun if properly cared for. Some bay horses exhibit dappling, which is caused by textured, concentric rings within the coat. Dapples on a bay horse suggest good condition and care, though many well-cared for horses never dapple. The tendency to dapple may also be, to some extent, genetic.

Bays often have a two-toned hair shaft, which, if shaved too closely (such as when body-clipping for a horse show), may cause the horse to appear several shades lighter, a somewhat dull orange-gold, almost like a dun. However, as the hair grows out, it will darken again to the proper shade. This phenomenon is part of bay color genetics, but usually not seen in darker shades of bay because there is less red in the hair shaft. (See: "Inheritance and expression," below)

Dark bay or "brown" horses often have lighter hair around the muzzle, eyes, flanks, and behind the elbow Kevin Tornado Zlosyn 2005.jpg
Dark bay or "brown" horses often have lighter hair around the muzzle, eyes, flanks, and behind the elbow

There are many terms that are used to describe particular shades and qualities of a bay coat. Some shade variations can be related to nutrition and grooming, but most appear to be caused by inherited factors not yet fully understood.

The palest shades, which lack specific English terminology found in other languages, are called wild bays. Wild bays are true bays with fully pigmented reddish coat color and black manes and tails, but the black points only extend up to the pastern or fetlock. Wild bay is often found in conjunction with a trait called "pangare" that produces pale color on the underbelly and soft areas, such as near the stifle and around the muzzle. [1]

Bay horses have black skin and dark eyes, except for the skin under markings, which is pink. Skin color can help an observer distinguish between a bay horse with white markings and a horse which resembles bay but is not.

Ambiguity of "brown"

Some breed registries (including the Jockey Club Thoroughbred registry) use the term "brown" to describe dark bays. However, "liver" chestnuts, horses with a red or brown mane and tail as well as a dark brownish body coat, are also sometimes called "brown" in some colloquial contexts. Therefore, "brown" can be an ambiguous term for describing horse coat color. It is clearer to refer to dark-colored horses as dark bays or liver chestnuts.

However, to further complicate matters, the genetics that lead to darker coat colors are also under study, and there exists more than one genetic mechanism that darkens the coat color. One is a theorized sooty gene which produces dark shading on any coat color. The other is a specific allele of Agouti linked to a certain type of dark bay, called seal brown. The seal brown horse has dark brown body and lighter areas around the eyes, the muzzle, and flanks. A DNA test said to detect the seal brown (At) allele was developed, but the test was never subjected to peer review and due to unreliable results was subsequently pulled from the market. [2] [3]

Effect of gray gene

Some foals are born bay, but carry the dominant gene for graying, and thus will turn gray as they mature until eventually their hair coat is completely white. Foals that are going to become gray must have one parent that is gray. Some foals may be born with a few white hairs already visible around the eyes, muzzle, and other fine-haired, thin-skinned areas, but others may not show signs of graying until they are several months old.

This horse is bay despite the fact that its black legs are masked by tall white markings Clydesdale-busch-gardens c.jpg
This horse is bay despite the fact that its black legs are masked by tall white markings
A very dark bay horse might appear to be almost black MaclintockVpic2.jpg
A very dark bay horse might appear to be almost black
A liver chestnut is distinguished from a bay by a lack of black points. The mane and tail are the same color as the body, or lighter. Mangalarga Paulista.jpg
A liver chestnut is distinguished from a bay by a lack of black points. The mane and tail are the same color as the body, or lighter.

Colors confused with bay

Detail of a dark bay horse, showing lighter hairs around the eye HorseEye c.jpg
Detail of a dark bay horse, showing lighter hairs around the eye
Detail of a black horse, showing solid black hairs around the eye HorseEye2 c.jpg
Detail of a black horse, showing solid black hairs around the eye

Bay-family colors

Traditionally, bay is considered to be one of the "hard" or "base" coat colors in horses, although genetically the simple base coat colors, based on the presence or absence of the extension gene ("E" or "e", respectively), are chestnut and black. Bay is the result of the agouti gene acting upon a black base coat. The effects of additional equine coat color genes on a bay template alter the basic color into other shades or patterns:

Genetics

Bay foals, like this one, sometimes have pale hairs on their legs and in their mane and tail until they shed their foal coats Poulain de race ardennaise 1.jpg
Bay foals, like this one, sometimes have pale hairs on their legs and in their mane and tail until they shed their foal coats

The various shades of bay may be genetically produced by multiple factors, but a simple explanation of bay genetics is that "red" color, seen in the chestnut horse, represented by the recessive "e" allele; and black color, represented by the dominant "E" allele, are the two most basic coat color genes. All other colors are produced by the action of additional alleles acting on these two base colors.

A bay horse carries both the Extension (E) allele and a suppression gene known as the agouti gene (A).The agouti gene, dominant over the black gene, limits or suppresses the black coloring to black points, allowing the underlying red coat color to come through. Unlike other types of "point" coloring, such as that seen in Siamese cats, the black points characteristic of bay coloring are not produced by a dilution or albinism gene.

Because the extension (E) gene and agouti (A) gene can be either heterozygous or homozygous, the extent to which a bay passes on its color varies widely from one horse to another depending on its genotype and that of its mate. Also, a chestnut may carry the Agouti gene, which will be "masked" or not manifest until the horse is bred to a horse with the E allele and produces offspring with both genes.

Inheritance and expression

The faint countershading dorsal stripe seen on some bays. This is not the same as the primitive markings of a dun horse. Countershading stripe.jpg
The faint countershading dorsal stripe seen on some bays. This is not the same as the primitive markings of a dun horse.

The bay family of coat colors is dependent on two autosomal simple dominant genes: Extension and Agouti. The role of the Extension gene is to produce a protein called Melanocortin 1 receptor or Mc1r. Mc1r allows the black pigment eumelanin to form in hair. Closely tied to this process, the role of the Agouti gene is to produce Agouti signalling peptide Asip, which disables Mc1r, effectively allowing the red pigment phaeomelanin to "show through." However, this disabling does not occur throughout the coat; it occurs only in pulses on the body coat and not at all on the extremities or points.

If a horse does not possess a functional, dominant copy of the wildtype E-allele at the Extension locus, then Mc1r cannot be produced. Without this protein, the black pigment eumelanin cannot form in the hair. Such horses, having two copies of the recessive mutation, have eumelanin-free, phaeomelanin-rich coats; they are red, or chestnut. In summary, unless a horse has at least one functional E-allele, it cannot be bay.

Similarly, if a horse does not possess a functional, dominant copy of the A-allele at the Agouti locus, then Asip cannot be produced. Without Asip, eumelanin is unregulated and the coat is wholly black. The regulation of black pigment, though, is dependent on its presence in the first place; a horse with the recessive Agouti genotype aa is indistinguishable from any other genotype in a horse with a eumelanin-free coat. When eumelanin is present, it is restricted in varying degrees by the action of Asip.

The action of Asip can be observed in horses which have their winter coats clipped. When shaved close, the black tip is shorn off leaving the phaeomelanic bottom of the shaft. This produces a dull, orange-gold appearance on the body coat which is lost with the spring shed. This is not usually seen in dark bays, which have little red in the hair shaft.

Agouti

The cause behind the various shades of bay, particularly the genetic factors responsible for wild bay and seal brown, have been contested for over 50 years. In 1951, zoologist Miguel Odriozola published "A los colores del caballo" in which he suggested four possible alleles for the "A" gene. He described an order of dominance between the alleles and the associated phenotypes:

This was accepted until the 1990s, when a new theory became popular. [5] The new theory suggested that shades of bay were caused by many different genes, some which lightened the coat, some which darkened it. This theory also suggested that seal brown horses were black horses with a trait called pangare. Pangaré is an ancestral trait also called "mealy", which outlines the soft or communicative parts of the horse in buff tan.

The pale areas on this bay horse are due to the pangare trait Ardennerpaerd.jpg
The pale areas on this bay horse are due to the pangaré trait

The combination of black and pangaré was dismissed as the cause of brown in 2001, when a French research team published Mutations in the agouti (ASIP), the extension (MC1R), and the brown (TYRP1) loci and their association to coat color phenotypes in horses (Equus caballus). This study used a DNA test to identify the recessive a allele on the Agouti locus, and found that none of the horses fitting the phenotype of seal brown were homozygous for the a allele. [6]

Since 2001, the mechanisms of the variations within the "bay" category remain unclear. Ongoing research suggests that Odriozola's theories may have been correct, [7] evidenced by a parallel condition in mice. Mice have more than six alleles at the Agouti locus, including At which produces black-and-tan. [8]

It is still likely that to some extent, the "shade" of coat color may be regulated by unrelated genes for traits like "sooty", and that the phenotypes of sooty or dark bays/browns may overlap. [9]

See also

Related Research Articles

Roan (color)

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 Genetic color in horses

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.

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:

Cat coat genetics

Cat coat genetics determine the coloration, pattern, length, and texture of feline fur. Understanding how is challenging because many genes are involved. The variations among cat coats are physical properties and should not be confused with cat breeds. A cat may display the coat of a certain breed without actually being that breed. For example, a Siberian could wear point coloration, the stereotypical coat of a Siamese.

At right is displayed the color traditionally called liver.

Gray horse

A gray horse has a coat color characterized by progressive depigmentation of the colored hairs of the coat. Most gray horses have black skin and dark eyes; unlike some equine dilution genes and some other genes that lead to depigmentation, gray does not affect skin or eye color. 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 more prevalent as the horse ages as white hairs become intermingled with hairs of other colors. Graying can occur at different rates—very quickly on one horse and very slowly on another. As adults, most gray horses eventually become completely white, though some retain intermixed light and dark hairs.

Cream gene

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.

Champagne gene

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

The silver or silver dapple (Z) gene is a dilution gene that affects the black base coat color and is associated with Multiple Congenital Ocular Abnormalities. It will typically dilute a black mane and tail to a silvery gray or flaxen color, and a black body to a chocolaty brown, sometimes with dapples. 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 Genetics behind the equine 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.

Dun gene

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.

Chestnut (horse color) Horse coat color

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.

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.

White horse

A white horse is born white and stays white throughout its life. A white horse 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.

Black horse Horse coat color

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.

Smoky black

Smoky black is a hair coat color of horses in which the coat is either black or a few shades lighter than true black. Smoky black is produced by the action of a heterozygous cream gene on an underlying black coat color. Therefore, smoky black is a member of the cream family of coat color dilutions, and found in horse populations that have other cream gene-based colors such as palomino, buckskin, perlino and cremello. All smoky blacks must have at least one parent with the cream gene, and a smoky black can be verified through DNA testing. Smoky black has been mistaken for faded black, dark bay or brown, grullo or even liver chestnut.

Seal brown (horse) Hair coat color of horses

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.

Roan (horse) Horse coat color pattern characterized by an even mixture of colored and white hairs on the body

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.

Dog coat genetics Genetics behind dog coat

Modern dog breeds have a wide range of coat colors, patterns, textures and lengths. Dog coat color is governed by how genes are passed from dogs to their puppies and how those genes are expressed in each dog. Dogs have about 19,000 genes in their genome but only a handful affect the physical variations in their coats. And the usual rules apply—most genes come in pairs, one from the dog’s mother and one from its father. Genes of interest have more than one version, or allele. Usually only one or a small number of alleles exist for each gene. So, at any one gene locus a dog will either be homozygous, that is, the gene is made of two identical alleles or heterozygous, that is, the gene is made of two different alleles.

The agouti gene (ASIP) is responsible for variations in color in many species. Agouti works with extension to regulate the color of melanin which is produced in hairs. The agouti protein causes red to yellow pheomelanin to be produced, while the competing molecule α-MSH signals production of brown to black eumelanin. In wildtype mice, alternating cycles of agouti and α-MSH production cause agouti coloration. Each hair has bands of yellow which grew during agouti production, and black which grew during α-MSH production. Wildtype mice also have light-colored bellies. The hairs there are a creamy color the whole length because the agouti protein was produced the whole time the hairs were growing.

References

  1. Sponenberg, Dan Phillip (2003). Equine Color Genetics 2e. Blackwell. ISBN   0-8138-0759-X.
  2. "The Enigmatic Brown Horse - Color Genetics". Archived from the original on 2016-04-08.
  3. Understanding Equine DNA and Agouti, at PetDNAServicesAZ; via archive.org; archived February 27, 2015
  4. Castle, W.E.; W.R. Singleton (September 1961). "The Palomino Horse" (PDF). Genetics. 46 (9): 1143–1150. PMC   1210264 . PMID   13877241. Archived (PDF) from the original on 2008-09-05. Retrieved 2008-03-04.
  5. B. Kostelnik (2007). "Starting Point". The Horse Colors Site. Archived from the original on 3 March 2008. Retrieved 2008-03-04.
  6. Rieder S, Taourit S, Mariat D, Langlois B, Guerin G (2001). "Mutations in the agouti (ASIP), the extension (MC1R), and the brown (TYRP1) loci and their association to coat color phenotypes in horses (Equus caballus)". Mamm. Genome. 12: 450–5. doi:10.1007/s003350020017. PMID   11353392.
  7. Laura Behning (2008-02-05). "The Base Colors". Morgan Colors. Archived from the original on 2008-05-17. Retrieved 2008-03-04.
  8. Hustad, C.M.; W. L. Perry; L. D. Siracusa; C. Rasberry; L. Cobb; B. M. Cattanach; R. Kovatch; N. G. Copeland; N. A. Jenkins (1 May 1995). "Molecular Genetic Characterization of Six Recessive Viable Alleles of the Mouse Agouti Locus". Mammalian Genetics Laboratory. 140 (1): 255–65. PMC   1206552 . PMID   7635290. Archived from the original on 8 September 2008. Retrieved 2008-03-04.
  9. Nancy Castle (2008-03-01). "Brown/Bay Dun". Dun Central Station. Archived from the original on 10 March 2008. Retrieved 2008-03-04.