of which distributes the black pigment throughout the horse’s body. Another genetic modifier is a gene that masks the expression of non-allelic genes, called epistatic.
Traits in living organisms are divided into quantitative and qualitative. Quantitative traits can be measured—that is, they are the kind of trait where how much you have can vary: body weight, height, and the thickness of bones, for example. Quantitative traits are usually not the product of one gene, but instead are coded by several pairs of genes and have a so-called polygenetic inheritance.
Qualitative traits, on the other hand, are usually monogenetic or influenced by a single gene. The phenotype is either/or—that is, you have one variant of the gene or another that dictates how it manifests. Pigments and blood type are examples.
When it comes to genes working nicely together (or not), there are other terms to become familiar with and to aim to understand:
Penetrance is the ability of a gene to show itself phenotypically. It can be either complete (manifested in each individual that carries the gene) or incomplete (not phenotypically expressed by all carriers).
Pleiotropic action is when one gene is responsible for two or more phenotypic traits.
Some genes can lead to serious deviations from the norm, such as decreasing its viability (sub-lethal effect) or even leading to death (lethal effect). The loss of an animal due to an unfavorable genotype can occur either at the early stage of embryogenesis or sometime after its birth.
Some genes located on the same chromosome can be linked and transferred together to offspring. Their pattern of inheritance differs from that of unlinked genes.
A mutation is a change in the DNA sequence of a cell or its locus. The extent of mutations can range from a single or few nucleotides to entire chromosomes. Mutations lead to variation and ultimately the formation of alleles. The allele that exists in its original “normal” form in the species is called wild. Other alleles of the same gene are the product of mutations involving the wild allele.
With a basic grasp of the vocabulary I’ve introduced in this first chapter, you’ll be able to now appreciate the genetics of horse color as much as you appreciate the myriad colors themselves. In the next chapter we’ll examine over a hundred colors, how they manifest, and the genetic influences at play. Throughout you will find references to the color photographs you can view in the Color Photo Reference sections.
CHAPTER 2:
Horse Colors
Base Colors
As I mentioned in Chapter One: Introduction to Horse Color, in this book I am working with four base colors in horses: bay, black, seal brown, and chestnut. They are called “base colors” because any individual horse must have one of them; they are the basis for color effects of additional genes.
BLACK
Black color (Photo 1) is evident in the hair on the horse’s torso, head, legs, and also the mane and tail. The skin is dark gray, the eyes dark or hazelnut, the horn on hooves is pigmented, and the eyelashes are black. Faded black color usually develops under prolonged exposure to external factors such as excessive sunshine, harsh weather conditions, or increased sweating, but it may also develop spontaneously (Photo 2). The tips of faded black hair acquire a dirty-brown color, and the extent can vary. The head and legs of the faded black horse are always darker than the body, since the hair on these places is very short and less subjected to noticeable discoloration in comparison with longer hair on the body. In order to distinguish faded black from seal brown (see p. 12), it is necessary to look at the horse’s head, in particular the region around the muzzle and the eyes. If in these places the hair is reddish or dark brown, then the color is seal brown. Note: There are also so-called non-fading black horses but they are less common. Such horses do not become faded, even when exposed to the effects of external elements.
Black foals are usually born ashy color and the legs and stomach are lighter than the remaining areas of the body.
Black horses are less common than bay (see p. 12) or chestnut (see p. 13). This color is the preferred color in Friesians. Some breeds where black color is frequently encountered include: the Percheron, Shire, Russian Saddle Horse, Russian Trotter, Kabardin, Vladimir Heavy Draft, Karachai, and the Kladruber. At the same time, it is rare in other breeds—Arabians, for example.
BAY
Bay horses have red or brown color of the trunk, and parts of the head, legs, mane, and tail are black (Photos 3–7). The tips of the ears and outer rims are also black. The skin and hooves are pigmented, eyes hazel, and eyelashes dark brown. The separation of the shades of bay is sometimes difficult and very subjective.
A light bay horse has light brown body hair. It is encountered rarely, and may be difficult to visually distinguish from buckskin (see p. 17). Dapples are observed in such horses more frequently than in the usual bay.
A bay horse with a dark or almost black torso is called dark bay. Dark brown and black hairs are mixed on the torso, and the brightest are concentrated above the nostrils, around the eyes, into groin, on the stomach, on the upper inside of the legs, and on the torso near the elbows. Let me emphasize that the expression “brightest” in this context is relative—in the case of dark bays it can be brown or dark brown, as long as it is “brighter” than the hair on the trunk. This color is characterized by dapples and can be confused with faded black (see p. 11). The two can be distinguished by the color of hair around the eyes: in the black horse, it will be black, while in the dark bay there is a brown or reddish ring in this place.
Bay without countershading, with a saturated reddish tone of body hair, is called red bay. This color is fairly rare. Bay with an expressed golden gloss of body hair is called golden bay. It occurs in the Akhal-Teke breed, and less frequently in Budyonny and Karabakh horses.
Sometimes you may encounter the so-called “Wild” bay color. On this horse the zone of black hair on the legs is restricted to the lower part, below the hocks and knees (Photo 8).
Bay foals have a red or light brown trunk and pale, almost white legs, with hardly noticeable dark hair below the hocks and knees. These areas become dark with age. The underbelly of a bay foal is also whitish.
Bay is the most common color in the world and is inherent to the majority of breeds. Some where it is found frequently include: the Arabian, Thoroughbred, Shire, Clydesdale, Paso Fino, Peruvian Paso, Kabardin, German Sport Horse, Latvian Horse, Vladimir Heavy Draft, French Trotter, and it is characteristic of the Cleveland Bay.
