About Color (and ASD)
Base and Modified Colors
Mountain Horses come in most of the colors found in other breeds – including the “base” colors of black, bay, and sorrel or chestnut. These three base colors in turn may be modified by almost any of the other genes which modify the base colors, so that you find roans, duns, grays, palominos and buckskins, etc. The only colors you find very little of in Mountain Horses are the paint colors, as the two largest registries, RMHA and KMSHA, to date do not allow horses with excessive white.
In the Mountain Horse breeds, and primarily the Rocky Mountain Horse registry, the color known as “chocolate” with a flaxen mane and tail is very popular, and has become strongly associated with this horse. The five foundation stallions of the RMHA were chocolate, all sons of Tobe, himself chocolate in color. The “chocolate” color is also found in other breeds, most commonly in Icelandics, Shetland Ponies and Miniatures.
The chocolate color is caused by a color modifying gene – that is, a gene which modifies (in this case dilutes) another color. This gene has been named the “Silver Dapple gene” by geneticists. It is a gene that only dilutes the color black. It does not affect the color red (sorrel or chestnut). Therefore, a sorrel or chestnut horse can carry the gene and pass it on, but you cannot tell whether the horse carries the gene from looking at the horse, as there is no black on its body to modify.
The silver dapple gene is a dominant gene which produces an even more dilute effect if two of them are present in the animal. In these homozygous horses, the manes and tails are almost white, and the black on the body is lightened up to an almost gray color. If it is a “red chocolate” horse (silver dapple bay), the black “points” on the legs are so light that horse almost looks like a solid chestnut with a white mane and tail.
Color and the Eyes
Color genes are arranged on the chromosome strand very close to the eye genes. Often, during reproduction, groups of genes travel together in “clumps” and are said to be genetically linked. This can happen with eye and color genes. One example is the gene for the spots of the Appaloosa, which shows some linkage with the tendency to develop uveitis or “moon blindness”, an inflammatory disease of the eye.
Geneticists from Michigan State University have determined, in one of the largest equine studies ever done, that a gene or genes affecting the physical development of the eye is associated with the Silver Dapple gene. This syndrome – a collection of differences in the structure of the front part of the eye – is called ASD, short for Anterior Segment Dysgenesis. In the vast majority of cases, ASD causes the horse no problems – it is merely a collection of harmless differences from the norm in physical structure.
Most breeders of Mountain Horses have become convinced that the Silver Dapple and ASD genes are 100% linked – that is, where the horse has inherited a single silver dapple gene, it will also have a single ASD gene. If the horse is homozygous silver dapple, it is also homozygous for ASD. This appears to be true even if the eyes of a chocolate horse examine as completely normal, without any indication of even a single ASD gene. About 13% of chocolate horses do examine this way, and they are called “silent carriers.” This gene linkage is a color phenomenon, not a breed issue. Researchers have found the ASD-silver dapple link in all breeds with this color.
The Nature of ASD
ASD is a “semi-dominant” gene. When the horse inherits one gene only from one of its parents, there is only a very slight physical indication, while two genes will cause more differences. With one gene, you see small, harmless structures behind the lens called “cysts”. They have no effect on the horse or its ability to see whatsoever. This will also not change over time. Cysts can usually only be detected by an experienced veterinary ophthalmologist with special equipment. About 87% of horses that carry a single ASD gene will show “cysts.” The remaining 13% are “silent carriers,” and can pass on the gene to offspring to the same degree as horses whose genetic status is visible.
When a horse inherits two ASD genes, one from each parent, the horse will almost always show other differences in addition to the cysts. (This is the horse that in lay terms has become known as the “ASD” horse.) Many of these are easily seen if you know what to look for. Not all homozygous horses will have all of the various differences which are part of the ASD syndrome – most will exhibit only some of them.
It has now been determined by a separate study using sophisticated equipment that only one of the various ASD syndrome characteristics adversely affects the vision of the horse, and that is where the lens itself is out of proper position or is detached. This is also the only characteristic with a progressive component – the detached lens will usually develop cloudiness that worsens with time.
Of all homozygous horses, only about 10% will have the subluxated lens which affects vision – a very small percentage of the whole population. If you are buying a horse, you do want to make sure that your candidate does not have this particular characteristic. If it does not, then the horse should be perfectly fine for using purposes no matter whether it has one ASD gene, two ASD genes or none.
Color and Breeding
If you are intending to breed a horse, the ASD gene status of your horse is something you should know in order to select a mate that will minimize chances of a homozygous ASD foal, as it is only the homozygous foal that has any chance of having affected vision. Again, we believe that any "chocolate" or "red chocolate" horse will carry at least one of these genes, and until we have a DNA test for the presence of this gene, horses should be bred with this assumption, regardless of the results of a visual exam. A sorrel or chestnut horse may carry none, one or two copies of the ASD gene. If you are unable to determine the ASD gene status of your horse from its color and/or its parentage, a veterinarian trained in the techniques for detecting ASD should examine the horse.
A horse that does not carry any copy of the ASD gene, (again, we believe this includes true black and bay horses), can be bred to any other horse, even a homozygous ASD horse, without fear of producing an ASD foal. Homozygous ASD must come from both parents. The horse that does not carry a copy of the ASD gene is symbolized as "aa."
A “cyst only” horse, that is, a horse which carries only one copy of the ASD gene (symbolized as "Aa"), can produce any combination of normal "a" and ASD "A" genes in offspring, when bred to another Aa horse. The chances with each Aa-to-Aa mating are as follows:
25% normal (no ASD gene and no silver dapple gene-"aa")
50% “cysts” (a single ASD and a single silver dapple gene-"Aa")
25% homozygous ASD (two ASD and two silver dapple genes-"AA")
It is this last category only which is at risk for affected vision – 10% of the 25% will have the subluxated lens (or about a 2.5% probability altogether with this breeding.) The homozygous ASD horse is symbolized as "AA."
Whether or not to breed two "cyst" horses together must be an individual decision, based on the breeder’s ability to absorb and deal with the very slight chance of producing an affected foal. In terms of the breed itself, we are advised by experts at the University of Kentucky that the cyst-to-cyst breeding is one that should continue, so that we can continue to produce the 25% of completely normal "aa" horses the breed needs for its breeding stock. Also, as we have a large number of chocolate, "Aa" horses in our breed, we are advised that to discontinue breeding these horses to each other would eliminate sufficient horses from the gene pool that it could potentially leave us open for other genetic problems developing within the breed.
Chocolate will always be popular. But we have learned to limit the production and breeding of the homozygous ASD horse. When the AA horse is bred, it should be bred only to the "aa" horse that does not carry any silver dapple or ASD gene. Many smaller breeders also prefer to not breed the single gene “cyst” horse to another “cyst” horse, but will only breed normal to normal, or normal to cyst, so as to eliminate the chance of an ASD foal.
Often, serious breeders will breed Aa to Aa and take the chance of an ASD foal, if the horses are an advisable match for improving the breed in other ways. Again, if you are considering purchasing an ASD "AA" foal or horse, there should be no problem with its vision if a veterinary exam reveals that there is no subluxation of the lens.
Finally, an interesting footnote. Some breeders and geneticists believe that the silver dapple color is linked not only to the ASD gene, but also to some of the characteristic personality traits that have made the Mountain Horses so popular! If this is true, it may be another reason for continuing to breed for the single-gene chocolate horse.This information was stolen (oops, I mean borrowed) from http://www.triple-s-ranch.com/about_color.htm.
This site was last updated 03/09/08