The darkest version of chestnut color is called liver chestnut (Photo 16). The mane and tail can be yellowish, the same shade as the horse’s torso, or almost black. This color can be distinguished from dark bay by the body and legs—they are black in a bay (see p. 14). There is also a very rare shade of dark liver chestnut color, which can be confused with black. In this case the tail and mane can range from white, to vivid red, to the same color as the horse’s trunk. This color can be found in Morgan horses, for example. Such horses can be identified as chestnut with the aid of DNA analysis.
The hair of chestnut horses can also fade; in such cases, a horse may look like he is covered with dust.
Chestnut foals are born red color with whitish legs and underbelly. Guard hair is often light. The foals become darker with age.
Chestnut color is common, surpassed in frequency only by bay. It is prevalent in the Arabian, Thoroughbred, Hanoverian, Budyonny, and Karabakh breeds, and some breeds have a characteristic chestnut shade, including Suffolk Punch and Russian Don horses. Chestnut color is encountered extremely rarely in Friesians, Percherons, Cleveland Bay, Orlov Trotters, Exmoor ponies, and Andalusians.
INHERITANCE OF BASE COLORS
Base colors in horses (black, bay, seal brown, and chestnut) are controlled by two genes: Extension and Agouti. The gene corresponding to the Extension locus is called MC1R (melanocortin-1 receptor). The “Wild” allele of this gene, designated “E,” is dominant and codes for the intact, normal receptor for the melanocyte-stimulating hormone. Upon binding of the hormone, the receptor leads to the synthesis of the black pigment eumelanin in melanocytes.
A recessive allele of the receptor is designated “e,” and in the homozygous state the horse can produce only a defective receptor. As a result the melanocyte-stimulating hormone cannot activate the production of black pigment and the cells switch to synthesize the red/yellow pigment pheomelanin.
Thus, genotype “EE” or “Ee” determines the presence of black hair, and genotype “ee” produces red. A DNA test is available to test for the “e” (red factor) allele. With the aid of this test it is possible to determine if there is a chance to obtain foals with red color—for example, from a black horse. The absence of the “e” allele in the genotype (EE) means that a horse is homozygous for the dominant allele; the presence of allele “e” in a single copy (Ee) indicates that the horse is heterozygous. A horse homozygous for the recessive “e” allele (ee) can only have a chestnut base color.
The dominant allele “A” of the gene Agouti codes for an Agouti-signaling protein (ASIP), which is an antagonist of the melanocyte-stimulating hormone, neutralizing its action in some areas of the body. In these areas there isn’t any black pigment production and the hair is red. In sections in which Agouti-signaling protein is not produced (the mane, tail, lower part of the legs, and ear rims), the pigment synthesis is switched to the black eumelanin and the hair in these locations is black. Thus, dominant allele “A” leads to the formation of the bay color. The recessive allele “a” of Agouti results in the lack of production of ASIP, and in a homozygous state (aa) does not suppress the synthesis of black eumelanin throughout the entire integument. Therefore an “aa” horse, which also has the genotype “EE” or “Ee,” will have a black base color. A phenotype effect of the Agouti gene appears only in the presence of allele “E,” and directs the synthesis of eumelanin in necessary quantities. The Agouti gene has no effect in an “ee” homozygous horse because there is only the red pigment produced.
At present there is a DNA test for the “a” allele. It is possible to determine with this test, whether, for example a bay horse can produce a black foal. Interactions of the genes and alleles of Extension and Agouti are summarized below.
Table 1.
| Agouti | Extension | Color |
|---|---|---|
| AA, Aa | EE, Ee | Bay |
| aa | EE, Ee | Black |
| AA, Aa | ee | Chestnut (often) |
| aa | ee | Chestnut (rare) |
However there are additional alleles in these loci. An allele of Agouti designated “At” is responsible for the seal brown color and has an intermediate phenotypic effect between “A” and “a.” There is a hierarchy of domination (A > At > a), and a DNA test developed in 2009 helps breeders to distinguish the seal brown color from dark bay. Interactions of this allele with others in Agouti and Extension are summarized in the following table.
Table 2.
| Agouti | Extension | Color |
|---|---|---|
| AAt | EE, Ee | Bay |
| AtAt, Ata | EE, Ee | Seal Brown |
DNA tests show that the “At” allele is encountered much more frequently than it was thought previously. In the United States, Dr. Michal Prochazka (the translator of this text) detected this allele and developed the DNA test, but as of writing the information has not been published yet. Based on my research, “At” has been found in several breeds, including: the Thoroughbred, Quarter Horse, Paint Horse, Arabian, Morgan, American Miniature Horse, and in some British ponies. This allele was also found in six Przewalski horses.
The existence of another allele “A+” has been proposed, which is responsible for the “Wild” bay color. However, again as of writing, it has not been confirmed.
The Extension locus also has a third allele “ea,” which is recessive and present only in Black Forest Horses. In the homozygous state it leads to the formation of chestnut color—that is, phenotypically it is analogous to allele “e.” This can introduce confusion with the analysis of the red factor “e,” because a chestnut horse homozygous for the allele “ea” can be erroneously identified as homozygous for “E.” However, chestnut offspring will be produced upon crossing with another chestnut horse.
BODY DISTRIBUTION OF PIGMENT
Once I was asked why in horses pigment is frequently located on the periphery of the body. Apparently, it can be explained by the lower body temperature in these areas. An illustrative example is the color pattern in Siamese cats, which is characterized by a dark nose, ears, paws, and tail. The reason for this distribution of pigment is a temperature-sensitive form of tyrosinase, which facilitates the synthesis of dark pigment in the sections of the body with a reduced temperature. In horses, the existence of such a form of tyrosinase and its effect on the color has not yet been proven. However, Bowling and Ruvinski (2000) suggested that the reason for the outlying distribution of pigment in horses is indeed a biochemical mechanism, connected with a reduced temperature on the periphery of the body.
DOMINANT BLACK
Scientists have been examining the so-called dominant black color, which presumably may be controlled by a separate allele “ED”. So far its location is not known, but in the opinion of some experts, it can be located in the Extension