A scheme of coat color classification based on recognition of the effects of the alleles of seven genes provides the necessary rigor, and with training can be uniformly applied by anyone to define most of the common colors encountered in horses. However, there are only three pairs of alleles which define coat color within Arabian horses which greatly simplifies the discussion. It is much more complicated if you include duns, buckskins, pinto etc and is beyond the scope of this discussion.
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Genes are made up of allel pairs. If both alleles are identical, then the animal is said to be homozygous at that gene; if the alleles are dissimilar, then the animal is said to be heterozygous at that gene. Information about the homozygosity or heterozygosity for various genes can be inferred from information about parents and/or progeny and can be used for predicting the outcome of matings. For most of the alleles of horse coat colors one cannot tell by looking at an animal whether it is homozygous for any coat color gene.
Both sets of genes function simultaneously in the cell. Often when the gene pair is heterozygous, one allele may be visibly expressed but the other is not. The expressed allele in a heterozygous pair is known as the dominant allele the unexpressed one as the recessive allele. The term dominant is given an allele only to describe its relationship to related alleles, and is not to be taken as an indication of any kind of physical or temperamental strength of the allele or the animal possessing it. Likewise, possession of a recessive allele does not connote weakness.
For simplicity in constructing models, geneticists symbolize genes by letters such as A, B and so on. A dominant allele of a gene can be symbolized by an italicized capital letter, e.g., A, and the recessive by an italicized lower case letter, e.g. a.
In any animal expressing the dominant allele of a gene, it cannot be determined by looking at the animal whether the second allele is a dominant or a recessive one. The presence of a recessive allele may be masked by a dominant allele, which leads to the expression "hidden recessive." Dominant alleles are never hidden by their related recessive alleles.
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Gene G: Exclusion of Pigment from Hair
Everyone is familiar with the process of progressive silvering of human hair color in which the hair color of youth, such as blond, brunette or redhead, turns to gray or white with age. Horses show a similar phenomenon of hair silvering with age in a color called gray. In horses, gray is controlled by the dominant allele G.
A young horse with a G allele will be born any color but gray and will gradually become white or white with red or black flecks as an aged animal. Earliest indications of change to gray can be seen by careful scrutiny of the head of a young foal, since often the first evidence of the gray hairs will be seen around the eyes.
A gray horse will be either GG or Gg. A homozygous gray horse (GG) will always produce gray offspring. However, it is not possible to tell by looking at the horse whether it is homozygous for G. Since gray is produced by a dominant gene, at least one parent of a gray horse must be gray. A non-gray horse is denoted by gg.
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The first step for defining the coat color of a horse which is not grey is to decide if the animal has any black pigmented hairs. These hairs may be found either in a distinctive pattern on the points (such as legs, mane and tail), or black hair may be the only hair color (with the exception of white markings) over the entire body. If a horse has black hair in either of these patterns, then the animal possesses an allele of the E gene which contains the instructions for placing black pigment in hair. Geneticists symbolize this allele of the E gene E. The alternative allele to E is e. Allele e allows black pigment in the skin but not in the hair. The pigment conditioned by the e allele makes the hair appear red.
If an animal has no black pigmented hair, it has the genetic formula ee. Basically, an ee animal will be some shade of chestnut. Manes and tails may be lighter (flaxen), darker (not black), or the same color as the body.
Since the chestnut animal is not gray, its genetic formula is gg ee. When two chestnut horses are bred together (gg ee x gg ee) the offspring must be chestnut (gg ee).
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Gene A: Distribution of Black Pigmented Hair
The gene that controls the distribution pattern of black hair is known as A. The allele A in combination with E will confine the black hair to the points to produce a bay. Various shades of bay from dark bay or brown through mahogany bay, blood bay to copper bay and light bay exist. The genetics of these variations has not been defined. Any bay horse will include A and E in its genetic formula as well as gg. See Table 1 for all possible allele combinations which produce bay. The alternative allele a does not restrict the distribution of black hair and thus in the presence of the allele E of the E gene a uniformly black horse is produced. In Arabians, as in most breeds, the a allele is rare. Less than 2% of registered Arabians are black. Many black horses will sun-fade, especially around the muzzle and flanks and such animals may be called brown.
Neither A nor a affects either the pigment or its distribution in chestnut (ee) horses. Thus it is not possible to determine by examination of coat color which alleles of the A gene a chestnut horse has.
You may encounter the term homozygous black. This means that a black horse is defined by the allele pairs gg aa EE and can never produce a chestnut offspring.
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Possible allele combinations for bay, black and chestnut coat colors:
Chestnut: gg AA ee, gg Aa ee or gg aa ee. Most chestnunt horses are the first combination as they will not carry the black gene. The last combination, gg aa ee, can result from two black parents being bred to each other but each carrying the chestnut gene. The parents are each defined as gg aa Ee in this situation and their offspring can be gg aa EE, gg aa Ee or gg aa ee. If one of the black parents were homozygous black then it is not possible for the breeding to result in a chestnut foal.
Bay: gg AA EE, gg AA Ee, gg Aa EE or gg Aa Ee. Most bay horses are one of the first two combinations as they will not carry the black gene.
Black: gg aa EE or gg aa Ee. The first combination is refered to as homozygous black.
Summary of Alleles and Actions of Horse Coat Color Genes
Gene |
Allele |
Observed Effect of Alleles |
G |
G |
GG: Horse shows progressive slivering with age to white or flea-bitten, but is born any non-gray color. Pigment is always present in skin and eyes at all stages of silvering. |
| Gg: Same as GG. | ||
g |
gg: Horse does not show progressive silvering with age. | |
E |
E |
EE: Horse has ability to form black pigment in skin and hair. Black pigment in hair may be either in a points pattern or distributed overall. |
| Ee: Same as EE. | ||
e |
ee: Horse has black pigment in skin, but hair pigment appears chestnut. | |
A |
A |
AA: If horse has black hair (E), then that black hair is in points pattern. The A gene has no effect on chestnut (ee) pigment. |
| Aa: Same as AA. | ||
a |
aa: If horse has black hair (E), then that black hair is uniformly distributed over body and points. The A gene has no effect on chestnut (ee) pigment. |
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A horse with a mixture of white and dark hairs of any color is known as roan. The extent of the pattern can vary from a few hairs in the flank to extensive involvement of the body. In the most extensively roaned horses, a typical pattern seen is that in which the body is silver due to a high percentage of white hair, but the legs and head are dark due to the preponderance of non-white hair. The roaning pattern may be present at birth or may not be conspicuous until the first foal coat is shed. Generally, roaning is not a progressive silvering phenomenon as is gray, although often the summer coat may appear lighter than that of winter. Young gray horses can mistakenly be classified as roan, but information about the color of the parents may help clarify the situation, since a gray will have at least one gray parent.
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