kaykay said:
the important thing is as breeders we all need to be aware of it and be able to identify it. The one i saw at auction was one of the worst i had ever seen and she was just a yearling. I was shocked how many people walked by her and said how cute her bug eyes wereÂ
 I was also shocked how many reputable breeders that have been breeding for years didnt have any idea of what it was. It is without a doubt inherited so any horse carrying this should not be bred
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Yes...it is inherited, but the trick is knowing what horses carry it, as MANY Silvers do "carry" this gene without showing it. So, the only way you can be absolutely sure you aren't passing it on, is to never breed two silver-carrying horses together. And, THAT is another problem.....there are still SO many Miniature breeders who don't know how to identify "Silver" carrying horses!
Here is some interesting information on ASD....
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 “cystsâ€. 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."