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 locus. Similar phenomenon of the inheritance of dominant black color has been described and studied in other animals.
The presence of this allele leads to the formation of black color even in genetically bay horses. As a result the carrier of this allele has a black color that cannot be distinguished from ordinary black horses (EE aa or Ee aa). Thus, allele “ED” is epistatic with respect to allele “A.”
A foal with dominant black is similar to bay: brown with darker guard hair rather than the ashy color typical for ordinary black foals. The phenotype of the foal carrying dominant black becomes darker with age.
Diluted Colors
Next we will discuss color modifications determined by dilution mutations. Currently there are several separate dilution genes (for example, Cream, Silver, Champagne, Dun), and each has its own biochemical and molecular mechanism of action. More often it is possible to view color dilution as a weakening of pigmentation, and the molecular mechanisms of the action of some of these genes/mutations are still unclear.
COLORS DETERMINED BY CREAM DILUTION
Buckskin
A buckskin is a bay horse with one Cream Dilution allele (Ccr). Buckskin horses have a sandy yellow color of trunk, neck, and head, while the mane, tail, and lower parts of the legs are black (Photo 17). The nuances of body color are broad—from pale yellow or almost white to dark yellow. The lower part of the legs sometimes appears brown rather than black. The skin is pigmented but can be somewhat lighter than in a bay horse; the eyes are hazel or amber. Dapples are frequently observed.
I distinguish three varieties of the color, depending on the bay base: light buckskin, average buckskin, and dark buckskin. Light buckskin horses are characterized by sandy, sometimes almost white coloring of body. Average buckskin horses have a standard yellow body color, and as a rule, dark buckskins can have a significant amount of dark hair, concentrated on the upper back (countershading) in contrast to the lighter abdomen (Photos 18 & 19). The parts closer to the darker area tend to show dapples.
Newborn buckskin foals are typically light in color, tending to darken after they shed their baby hair. The lower part of the legs can be diluted or black, and with time the black zone tends to extend almost to the knees and elbows.
This color is encountered in many breeds: Lusitanos, Quarter Horses, Miniature Horses, Shetland Ponies, Welsh Ponies, Connemara Ponies, Australian Ponies, and Kinsky horses, for example. Light or dark buckskin color (with visible countershading) is common in the Akhal-Teke breed, and the Byelorussian Harness Horse frequently features very light-colored horses with an almost white trunk. Very rarely is the buckskin color found in Orlov Trotters, and only very recently was it discovered in Thoroughbreds.
Smoky Black and Smoky Seal Brown
In some countries the smoky black color is not considered its own entity and is not recorded in breeding documents, and rarely is referenced in the literature. Visually this color is difficult or even impossible to distinguish from faded black (see p. 11).
