"Chocolates"

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, Miniatures and some draft breeds. 

            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 diluted effect if two of them are present in the animal (one inherited from each parent).  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. 

            It has become common for people in the breeds to refer to only the homozygous horse, who carries two of the ASD genes, one from each parent, as an "ASD horse."  The horse that has inherited only one ASD gene from one parent, is referred to as a "cyst-only" horse.   These horses usually show only one of the characteristics of ASD, small structures behind the lens called "cysts".  The cysts cause no problems for the horse or its vision.

            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, and will have two of each gene.  This linkage appears to occur 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."  Silent carriers are usually heterozygous so they will pass on the ASD gene 50% of the time just like horses whose eyes show cysts. 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 

            The abnormal ASD gene is a "semi-dominant" gene.  When the horse inherits only one abnormal gene from one of its parents, there is only the very slight and harmless physical indication of cysts (or none if it is a silent carrier) behind the lens.  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 in an eye exam - that is, 50% of the time. 

            NOTE:  Please be aware that this distinction between the results of an eye exam and the actual genetic status of a horse as a carrier of the abnormal ASD gene is important to understand.  Unfortunately, many breeders or sellers of our horses either do not understand or do not properly explain that a "normal" eye exam is not in any way a guarantee that the horse does not carry the gene. There are a significant number of chocolate "silent carriers" that examine as normal but carry, and pass on, the ASD gene.  Some breeders may advertise a horse that has had a "normal" result on an eye exam as being "aa" - which is the genetic symbol for a horse that does not carry a dominant abnormal ASD gene ("A").  This is very misleading and detrimental to anyone who may want to breed the horse they are purchasing.  An eye exam can only confirm the presence of the ASD gene, but it cannot rule it out.  Until we have a genetic test for the presence of the abnormal ASD gene, which to date we do not, it must be assumed that all chocolate or red chocolate horses are carriers or "silent carriers" of at least one abnormal ASD gene, and will be able to pass it on in breeding.

            When a horse inherits two (abnormal) 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, that is, a characteristic that will get worse over time - the detached lens will usually develop cloudiness that worsens with time. This can only occur with the homozygous ASD horse.

            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.   

               The good news is that, except for the very small percentage of horses with subluxated lenses that will get more and more cloudy, the characteristics of ASD do not change over time.  They are physical, developmental differences that occurred in the womb.  What you (or your vet) see is what you will get, and continue to have, from the time the horse is born.  The changes from normal are not a disease process, cause the horse no pain, and will not come and go or appear at a later time.  You can know from a competent prepurchase exam what you are getting, then and in the future. 

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 or color of its offspring, a veterinarian trained in the techniques for detecting ASD should examine the horse.  Unfortunately, even an exam cannot eliminate the possibility of the presence of the ASD gene, but it may well confirm it.

            A horse that does not carry any copy of the ASD gene can be bred to any other horse, even a homozygous ASD horse, without fear of producing an ASD foal that may be at risk for vision loss.  Homozygous ASD  must come from both parents.  The horse that does not carry a copy of the ASD gene is symbolized as "aa," and cannot pass on the abnormal gene. This is the only horse that should be bred to a homozygous ASD horse, symbolized as "AA,"  and is a popular choice for breeding to the heterozygous, cyst-only horse. Remember, the truly genetically normal "aa" horse can never produce a horse with a vision problem from ASD, regardless of the genetic status of its mate.

            How can you know if a horse is truly genetically normal, if there are silent carriers in which the presence of the gene cannot be detected?  The only clue breeders have had since the ASD-silver dapple relationship was discovered, is the color of the horse.  Chocolates and red chocolates are assumed to be carriers; blacks and bays are assumed to be normal.  Here we have a true mystery, what I call "the mystery of ASD."  While geneticists have stated that they have seen "cysts" in horses that appear bay or black, and have warned breeders that they cannot rely on color to make breeding decisions that will avoid homozygous ASD, breeders have had to rely on color, as that is all they have had to use in the absence of a genetic test and considering the problem that eye exams do not detect all carriers.  And over the last ten years of countless thousands of horses bred, it has proved to be a very reliable tool.  Breeders in the field who want to avoid homozygous ASD, breed chocolates to black or bay horses, and do not get ASD.   It has been as simple as that.  Chestnut horses are the only unknown, but chestnut horses are underrepresented in our breeding population due to the color-consciousness of our breed fanciers.

          

  So let's get a little more specific.  A "cyst only" horse, that is, a horse which  carries only one copy of the ASD gene (symbolized as "Aa"), presents the most issues for breeding.  If bred to an "aa" horse that does not carry the gene (we assume this includes true blacks and bays), you have no chance of a homozygous "ASD" foal.  But often, breeders wish to breed chocolate to chocolate, both being single-gene Aa carriers.  This can produce any combination of genes in the offspring, from completely normal "aa" to homozygous ASD "AA".  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 only this last category 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.) 

            Whether or not to breed two heterozygous "cyst-only" 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, far worse, genetic problems developing within the breed. 

            Chocolate will always be popular. The abnormal ASD gene will never be eliminated from our horse population as long as we desire to produce this color.  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 - the true black or bay horse. 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.