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.

Like bay, the seal brown color lacks the non-agouti mutation that would create a fully black horse.^[1] The genetics behind seal brown are not known, but some think it is caused by an allele of agouti called A^t.^[2] A DNA test said to detect the seal brown (A^t) allele was developed, but the test was never subjected to peer review and due to unreliable results was subsequently pulled from the market.^[3][4]

The similar dark bay coat color, which also features black points and a dark body, differs from seal brown by the absence of tan markings. Another mimic is the liver chestnut, an all-over dark brown coat including mane and tail, that is sometimes confused with seal brown. However, true seal browns have black points characteristic of all bay horses, while liver chestnuts do not.

Opinions vary on what constitutes a true seal brown as distinct from dark bay. 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."^[5] 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.^[6]

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.^{[7][8][9][10]} 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.^[7]

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."^[5] 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.^[8]

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

• Brown Buckskin, is a result of the dilution effect of a single copy of the cream gene. Sometimes called smoky brown. The black areas of the seal brown coat are unaffected or slightly lightened, while the reddish areas are more golden. These should not be confused with traditional buckskins.^[16]
• Sable champagne, a result of the dilution effect of the champagne gene. Like all champagnes, sable champagnes have hazel eyes and pinkish, freckled skin. The coat is a flat, diluted grayish- or purplish-brown, somewhere between the warm pumpkin tones of the bay-based amber champagne, and the cool purplish tones of the black-based classic champagne.^[17]
• Brown dun, a result of the dilution effect of the dun gene. Like all duns, brown duns have conspicuous primitive markings including at least a dorsal stripe and darker points. The primitive markings of brown duns are black, and the coat color is somewhere between the slate gray of a grulla and the tan of a bay dun.^[18]

The genetics behind seal brown are not known.

Since 1951, it has been proposed that seal brown was caused by an allele of the agouti gene, given the name a^t.^[20] This is based on the many other species where similar black-and-tan patterns are caused by alleles at the agouti locus.^[2] One genetics lab offered a test for seal brown in 2009,^[21] but the underlying studies were not peer-reviewed and the test was pulled from the market due to inconsistent results.^[3]

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.^[22] This theory prevailed until the 1990s, when discoveries of similar conditions in other species provided alternate explanations.

Both Dark Bay horses, which have a black mane, tail, and legs with a dark reddish brown or sooty coat, and seal brown horses, which have very dark brown coats in addition to black "points", with reddish or tan hairs around their muzzle, eyes, elbows, and flanks have one of two genotypes at the Agouti locus: A/A or A/a.^[27] 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. These horses are often mistaken for seal browns or dark bays. Horses which do not undergo such fading are now usually called “non-fading” black, though other terms were used in the past.^[28] However, though hypothesized, there does not appear to be a separate “non-fading” allele for black, either.^[27] 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.^[29] This type of dosage-dependent behavior was not observed with Agouti.^[30]