Bay (horse)

For the community in England, see Bay Horse.

Bay is a hair coat color of horses, characterized by a reddish 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. The most prevalent colors besides bay are black - a wholly black coat - and chestnut - a wholly red coat.

The black areas of a bay horse's coat are called black "points", and without black points, a horse cannot be bay. The color of these points may be obscured by the various white patterns found in horses, however such markings do not alter a horse's classification as "bay." True white coat markings grow from pink areas of skin. This can help an observer distinguish between a horse which resembles bay but is not, and a bay horse with white markings.

A foal born bay may, if it has a gray parent, turn gray itself. However, gray can be regarded as a type of pattern and many horse registries include the original color of gray horses in their documentation. Also worth noting is that bay foals typically do not have black legs at birth.

Terminology

There are many terms that are used to describe particular qualities of a bay coat. The pigment in a bay horse's coat is rich and fully-saturated, as opposed to dilute buckskins. 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 horses never dapple.

Bay coats also vary in shade. These variations can be related to nutrition and grooming, but primarily appear to be caused by inherited factors not yet fully understood. The palest shade, which lacks a specific English term equivalent to those found in other languages, is called wild bays. The black points on a wild bay only extend up to the pastern or fetlock, though the mane and tail are still black. Wild bay is often found alongside pangare, a trait that produces pale underparts and soft areas, such as just aft the stifle and around the muzzle.

The darkest shades of bay are often called seal brown or simply brown. In other languages, the term for this color is the equivalent of "black-and-tan." Seal browns may be so dark as to have nearly wholly-black coats, with brown visible only under the eyes, around the muzzle, behind the elbow, and in front of the stifle.

In between these two extremes is the rich red blood bay as well as darker shades sometimes called mahogany bay, black bay or dark bay.

Inheritance and expression

See also: Introduction to genetics

The bay family of coat colors is dependent on two 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 unreglated 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:

• a, the least dominant, must be homozygous to be observed and is responsible for unrestricted black coat (non-agouti black),
• A^t, only visible in the homozygous form or when paired with a, is responsible for the black-and-tan seal brown coat,
• A, visible when homozygous or when paired with a or A^t, is responsible for the standard bay coat,
• A+, dominant, is responsible for the wildtype wild bay coat.

This was accepted until the 1990's, when a new theory became popular. 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. Pangare is an ancestral trait also called "mealy", which outlines the soft or communicative parts of the horse in buff tan.

Black and pangare was dismissed as the cause of seal 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, thus none were black.

Since 2001, the mechanisms of the variations within the "bay" category remain unclear. Ongoing research suggests that Odriozola's theories may have been correct, evidenced by a parallel condition in mice. Mice have more than six alleles at the Agouti locus, including A^t which produces black-and-tan.

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 and seal browns may overlap.

Bay-family colors

Bay is considered to be one of the "hard" or "base" coat colors in horses. The two others in this category are chestnut and black. The effects of some of the other equine coat color genes on a bay template include:

• Buckskin: Bay horse heterozygous for the dominant creme (CCr) allele. The black pigment remains largely unchanged, but any red pigment in the coat is diluted to gold, producing an overall bronze effect.

• Perlino: Bay horse homozygous for the dominant creme (CCr) allele. Both black and red pigment are diluted to some shade of creme, though the formerly-black points often have a stronger reddish cast. The skin is a slightly pigmented pink and the eyes are blue.

• Bay dun: Bay horse with at least one dominant dun allele. Red and black pigment at the extremities remains largely unchanged, but on the body black pigment is diluted to slate and red pigment is diluted to a dustier shade. The effect is similar to buckskin, but the coat of a bay dun is a flatter tan rather than bronze, and all bay duns have some form primitive markings.

• Amber champagne: Bay horse with at least one dominant champagne allele. Black pigment is diluted to warm brown and red pigment to gold. The effect is similar to buckskin, but the points of an amber champagne do not remain black, and the skin is curiously freckled. Amber champagnes also have hazel eyes, rather than brown.

• Silver bay: Bay horse with at least one dominant silver (Z) allele. Red pigment is unaffected, but black pigment in the short coat is diluted to dark, flat, brown-gray while the longer hairs are diluted to silver. The overall effect is that of a chocolate-colored horse with a paler mane and tail.

• Bay roan: Bay horse with at least one dominant roan (Rn) allele. White hairs are interspersed throughout the body coat, but not the extremities. This color was formerly called "red roan."

• Bay pinto: Bay horse with any number of white spotting genes, including but not limited to tobiano, frame, splash, sabino1, rabicano, and so on. The pattern has no bearing on whether or not the horse is bay.