| Home | Posted Nov 30, 2006 | |
Color does not make the horse, but it is hard to dispute that a desirable color increases market value. Rather than being at the mercy of Mother Nature, it is possible through the understanding of a few genetic rules, to be able to tip the scales in the favor of producing the color you want. To be able to plan matings to produce a certain color, it is essential to grasp the concept of dominant versus recessive inheritance. All genes are expressed in terms of their dominant or recessive forms (alleles). Dominant means that if the horse inherits the dominant form of the gene it will be expressed. A recessive allele can be carried by a horse and will not be expressed if it is masked by a dominant form of the gene. All the "icing on the cake" colors-silver dapple, cream, champagne, roan, gray, dun and even the tobiano spotted pattern, are controlled by dominant alleles for the respective gene.
Because a horse inherits an allele for any gene from both parents, every horse has two alleles at each locus (the location of the gene on a chromosome), one from the sire and one from the dam. When identical alleles are inherited from each parent at a given locus the horse is said to be homozygous. The horse may be homozygous for the dominant form of the allele or homozygous for the recessive form of the allele. If different alleles are present, the horse is heterozygous. A horse may be homozygous at some loci and heterozygous at others. Knowing the color gene makeup of each parent, potential foal colors can be predicted.
Of particular interest in this breed is the production of the chocolate coat color, also known as silver dapple, under the genetic control of the Silver Dapple locus. The effect of the dominant allele Z is to alter the black pigment. Therefore only horses expressing black pigment can be silver dapple. Thus it is important to understand the genetics of both the Silver Dapple locus and the locus controlling expression of black pigment, which is the Extension Locus. The Extension Locus allele that allows black pigment to be expressed is designated E. The alternative allele is e which prevents black pigment from being expressed. A horse homozygous for e will not have any black pigment and will be a chestnut/sorrel regardless of whether or not it also has Z for silver dapple. Therefore two chestnut horses (both ee) cannot produce anything but chestnuts! A horse with the genotype Ee or EE will express black pigment, in the mane and tail and on the legs in the case of a bay horse and all over the body in the case of a black horse. A black horse with one copy or two copies of Z will be the classic chocolate color. A bay horse with the Z allele is a red silver (red chocolate). Similarly the Z allele can be expressed in any horse with at least one E at the Extension locus such as buckskins, zebra duns, browns and grullos. Horses homozygous for Z are said to have whiter manes and tails that are less prone to darkening with age as occurs with horses heterozygous for Z. The association of the eye disorder anterior segment dysgenesis (ASD) with the Silver Dapple locus makes it risky to breed for the homozygous Z genotype.
Since true chocolates are horses that are genetically black, the gene that controls where black pigment is expressed must also be considered. This locus is known as Agouti and has two alleles A and a. The A allele restricts the expression of black pigment to the points, and results in a bay (when E is also present!). Remember the Extension locus allows black pigment to be expressed at all- Agouti determines where that pigment is expressed. Because A is dominant, a horse only needs one copy to be a bay. If the horse is homozygous for a and possesses at least one E, it will be black. So a chocolate will be homozygous for a, and have at least one E and one Z allele. A red chocolate will have the same genotype, but have at least one A instead of being homozygous for a. So what's a breeder to do if breeding for color is important to their program? Some determinations of color genetics can be made just by looking at the horse. For example, a chestnut horse (no flaxen mane and tail) we know must be ee. You cannot tell whether it is also black or bay at the Agouti locus, or whether it caries the silver dapple allele (Z). Fortunately for all the other cases that might be murky, genetic tests are available that detect the actual mutations behind the alleles. Tests are available for both the E locus and the Agouti locus. Therefore you can tell whether a horse is homozygous for E, which means it will never produce a chestnut, or homozygous for a, meaning it is genetically black. There is no need to test a chestnut horse for the E locus, but you may want to test to see if it is genetically black. If so, it can be bred to a homozygous E, homozygous black (aa), and a black foal will always be produced (Eaa). Two horses that are EEaa, when bred together will always produce blacks. If one horse is EEaaZz, 50% of the foals will be chocolates and 50% will be black.
Some chocolates may be difficult to distinguish from liver chestnuts or red chestnuts; in such cases the presence of an E will be diagnostic that the horse is actually a silver dapple. Remember that one parent must also be a silver dapple, unless the silver dapple is being masked in a chestnut horse. Unfortunately there is not yet a genetic test available for the Silver Dapple locus. There are scientific groups working on this and a test is probably not far off. In the meantime, judicious use of the available genetic tests and a basic understanding of color genetics can allow breeders to control foal color through planned matings.
Kathryn T. Graves, PhD is Director of the Equine Parentage Testing and Research Lab at the University of Kentucky. She has been with the program since 1986 supervising the Red Cell Typing and Reagent production laboratories. She has bred, trained and shown American Quarter Horses and Paint Horses since 1980.
Address: Equine Parentage Testing and Research Lab
University of Kentucky
101 Animal Pathology Bldg.
Lexington, KY 40546-0076