Seal brown (horse)

Identification

The research behind the classification of seal brown as distinct from dark bay is quite new, and as a result, opinions vary on what constitutes a true seal brown. In Equine Color Genetics, Dan Phillip Sponenberg wrote "In general, all dark colors with black points that are lighter than black but darker than bay are called brown." In this text, he classifies black-pointed, clear reddish coats of any shade as bay, and black-pointed coats of any shade with black countershading as brown. These definitions, while precise, are no longer accurate in light of current research.

True seal brown is best described as a black or nearly-black coat with reddish or tan hairs on the "soft parts": the muzzle, eyes, inner ears, underbelly, behind the elbow, and in front of the stifle. Like other coat colors, seal browns can range in shade. The very darkest are just about black except for their tan areas. Lighter examples are easily confused with dark bays. The mane, tail, and legs are always black.

Terminology

Non-horse people often refer to many horse coat colors as "brown," in particular the bay color. Among horse aficionados, a common assessment is that "...[the term] is only used by people with one horse or with two hundred." The implication is that lay observers will refer to a horse's coat color to be "brown" due to a lack of vocabulary, and those discussing large populations of horses will use "brown" out of a need for a more specific vocabulary. The term "seal brown" is unlikely to be part of a novice's repertoire and is therefore preferable when discussing this specific coat color. This coat color is, illuminatingly, called "black and tan" in some languages.

"Brown" but not seal brown

In the most simple terms, the vast majority of horses are indeed some shade of brown, but not "seal brown." Such coat colors include:

Seal browns on paper

Not all breed registries or studbooks recognize seal brown as a distinct coat color. The American Quarter Horse Association and American Paint Horse Association both recognize "brown" as a separate category, while the Arabian Horse Association labels all non-black, black-pointed shades "bay."

Still other registries, such as The Jockey Club which registers Thoroughbreds and Appaloosa Horse Club, offer the designation "dark bay or brown" to cope with the ambiguity in terminology and identification. Among historically German breeds and registries, the term rappe indicates a black horse, braun is bay, while dunkelbraun indicates dark bay and schwarzbraun indicates seal brown (literally black-brown). In France, seal brown horses are recognized among the "black coat family".

Related coat colors

The presence of other coat color genes can modify a seal brown coat. The seal brown family includes:

Genetics of seal brown

The appearance of a horse's skin, eye, and coat-color is determined by pigment chemicals called melanins. Two types of melanins are used by mammals, such as horses and humans: eumelanin, which is visually black to brown, and phaeomelanin, visually red to yellow. Specialized cells in the skin and eyes, melanocytes or pigment cells, produce melanins and deposit them into the skin and hair using complex chemical reactions. The instructions for these chemical reactions are genetically encoded as DNA, and are therefore inherited. To be seal brown, a horse must have at least one copy of the functional MC1R gene (E/E or E/E^e) and must have one of the following genotypes at the Agouti locus: A/A or A/a.

One protein with an important role in eumelanin production is Melanocortin 1 receptor (MC1R). The gene which encodes a functional MC1R protein occupies the Extension locus, or chromosomal position, and is symbolized by the capital E. A mutation, or change, in the equine MC1R gene, that resulted in a non-functioning MC1R protein, was identified in 1996. This form of the gene is symbolized by the lowercase e or sometimes E^e. Since each horse has two copies of the MC1R gene, one from each parent, horses with one "broken" copy (e) can still produce eumelanin in the hair. However, if a horse has two copies of the "broken" allele and therefore no copies of the functional E allele, the horse is completely incapable of depositing black pigment in the hair. Such horses are chestnut, or at least, are not black, bay, or seal brown.

The gene with the greatest role in the seal brown coat phenotype is Agouti signalling peptide (ASIP) or simply Agouti. The functional Agouti signalling peptide (protein) acts as a switch between red-yellow phaeomelanin and black-brown eumelanin. It is ASIP that is responsible for the alternate banding of dark and light on animal hair, although it is also responsible for whole-body effects. ASIP attaches to MC1R molecules to temporarily prevent the latter from continuing the production of eumelanin, and so phaeomelanin is produced in its stead. An individual horse can only have two copies of the Agouti gene, with the following known alleles or options:

Theories of the existence of additional alleles on the Agouti locus have been proposed, but have not been verified. These are

The A and A^t alleles cannot be distinguished by DNA testing. a test had been developed, but the underlying studies were not peer-reviewed and the test was pulled from the market due to inconsistent results Equine geneticists have mapped ASIP and found only two forms, dominant A and recessive a. While there may be a genetic reason behind the seal bay or brown and wild bay phenotypes, the cause is not located at Agouti.

Former theories about the genetics of seal brown

An early version of the currently-accepted equine Agouti gene theory was first presented in 1951 by Miguel Odriozola in A los colores del caballo, subsequently reviewed by William Ernest Castle in Genetics. This theory prevailed until the 1990s, when discoveries of similar conditions in other species provided alternate explanations.

Black and pangaré

For a period, the seal brown phenotype - black or near-black coat with tan or red hairs on the soft areas - was described as a true black coat affected by pangaré, or mealy-factor. Pangaré is a quality common to the Przewalski's horse and so-called primitive horse breeds such as the Exmoor Pony. The trait is characterized by pale hairs, typically off-white to light tan, around the eyes, muzzle, and underside of the body. By this definition, seal browns would possess the recessive a/a Agouti genotype. Dan Phillip Sponenberg described seal browns in this way, suggesting that dark browns - by his definition, sooty bays - could be distinguished from seal browns by a quality of redness, stating "[the] mealy effect generally is lighter and more yellow than residual nonblack areas (which tend to be redder) on very sooty horses."

This theory was discarded when the equine Agouti gene (ASIP) was sequenced in 2001, and all horses fitting the seal brown phenotype did not possess the homozygous recessive a/a Agouti genotype.

Tyrosinase-brown

Tyrosinase-related protein 1 (TYRP1) is a protein involved in melanin synthesis, and is encoded by the TYRP1 gene, also called the brown (b) locus. In humans, mutations in the TYRP1 gene account for variations in "normal" skin, hair and eye coloration, as well as types of clinical Albinism. Mutations in the TYRP1 gene of other mammals result in various reddish-brown coat color phenotypes: Brown in mice, Chocolate in cats, Chocolate in dogs, and Dun in cattle.

The phenotypes associated with TYRP1 mutations are typically rufous or chocolate rather than the black-dominated coats of seal brown horses, and usually result in pinkish-brown skin and light eyes. This is not the case for seal brown horses, and the role of TYRP1 in seal brown was ruled out after it was sequenced in 2001.

Extension-brown and dominant black

The allure of a pure black coat on a horse has struck horse breeders for centuries, resulting in all-black breeds like the Friesian horse. The breeding of pure black horses is attended by two problems: some black coats fade with exposure to light and sweat, and breeding two "black" horses together would sometimes produce non-black horses. In some cases, faded true black horses have lighter coats than the darkest near-black horses.

To account for this, W.E. Castle postulated that there was a third allele at the Extension locus: E^D or "dominant black". Based on the existence of such conditions in other animals, Castle suggested that the dominant black gene (E^D) would override the "points" pattern of dominant Agouti (A) and produce black or near-black horses, which could then go on to have bay offspring. The implication was that the seal brown coat color, which is often quite nearly black, could be produced by this allele.

Similarly, D.P. Sponenberg suggested an Extension-brown (E^B) allele, dominant over the wildtype E. He described an allele responsible for black countershading, or sootiness, which would distinguish all shades of brown from all shades of bay.

Both theories were laid to rest after the characterization of the equine MC1R or Extension, which showed no such alleles. However, it remains likely that a genetic control for sootiness does exist.

Dark bay vs. seal brown

Bay horses, which have a black mane, tail, and legs with a mixture of red and black hairs on the body coat, and seal brown horses, which have primarily black coats in addition to black "points", with reddish or tan hairs around their muzzle, eyes, girth and flanks have one of two genotypes at the Agouti locus: A/A A/a. Both coat colors exhibit a broad range of potential shades due to a variety of factors including the bleaching or fading of black hair, nutrition, and the presence of sooty or countershading factors.

Many black horses fade, sunbleach, or otherwise undergo a lightening of their coat color with exposure to sunlight and sweat. Horses which do not undergo such fading may be called jet black, raven black, non-fading black, or sheer black. However, though hypothesized, there does not appear to be a separate “non-fading” allele for black, either. Mineral and vitamin deficiencies can also contribute to a lighter coat, similar to sunbleaching.

Black-pointed horses that are not uniformly black often exhibit a trait called sootiness. A sooty coat exhibits a mixing of black or darker hairs more concentrated on the dorsal aspect (top) of the animal, and less prevalent on the underparts. Sootiness is thought to be a form of countershading. Horses without any sootiness are termed "clear-coated". Sootiness can be minor or quite extensive, and often includes dappling. Dark bay horses are typically sooty. The difference between the top-down distribution of the sooty trait and the lighter soft areas of a seal brown can also be difficult to distinguish from one another. The team of French researchers who developed the DNA test for the recessive a allele also discussed the possibility that Extension might be dosage-dependent. They found a statistically significant tendency for lighter bays to be heterozygous for the dominant, wildtype Extension allele (E/e, also written E⁺/E^e) while darker bays were more often homozygous (E/E). The authors acknowledged that other factors could play a role, and that the claim needed to be studied on a greater scale. This type of dosage-dependent behavior was not observed with Agouti.