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As you can see, if you breed a carrier with a completely clean horse, you will not receive a foal with ASD. There is a 50% chance your foal will be a carrier, though, so you will never have a guarantee of a completely clean horse if one of the parents is a carrier. If you want to breed a carrier, it is vital you make sure the partner is (aa), otherwise the foal has a chance of being (AA).
Once again I stress that ASD is not a widespread problem. It is a problem you will be forced to confront if you purchase a heavily inbred horse, or a chocolate horse. The chances of a chocolate horse with a flaxen mane and tail having ASD or being a carrier for it has been shown to be about 80%. Purchasing a carrier is almost unavoidable when you are looking at chocolate horses, though it is possible to find a (aa) chocolate horse by avoiding certain bloodlines and checking breeding history to minimize chances of an inbred background. If you are looking to purchase Mountain Horses of colors other than chocolate, ASD is almost a non-issue.
I do want to relate one more fact about ASD to anyone who may consider purchasing a Mountain Horse in the future. The KMSHA and RMHA have both grandfathered existing certified breeding stock into their registry. The KMSHA required from January 2, 1998 and on that any horse applying to be certified for breeding be veterinarian checked and cleared of ASD (AA). The RMHA recommended breeding stock be checked for ASD, but won't disqualify breeding stock based on ASD status. Though this may sound like a beneficial move to stop the spread of ASD in the future, it is by no means a fail-proof plan. Why?
First and most obviously, the grandfathering of all existing certified to breed stock: by accepting all existing "certified to breed" stallions and mares afflicted with ASD as qualified breeding stock, the registry allowed corrupted horses to continue passing on bad blood for years to come. My next problem with the prior guidelines? Before a new horse could be certified for breeding, it must have been cleared of (AA) – full-blown ASD. Will that stop ASD entirely? Not at all - check the comparisons above again where a carrier (Aa) was bred to another carrier. You'll notice there is a 25% chance that two carriers will create a full-blown ASD foal. We have a case where applicants for registrations and certifications to breed needed to deal with new hassles and expense (bloodtyping and vet checks), yet ASD would still not be stopped due to the grandfather clause and the 25% chance of two carriers producing an ASD offspring.
I don't bring my criticism of the policies in the registries here to attack the respective registries, but rather to show you that their policies do not ensure "certified to breed" mares/stallions are safe breeding choices. If you breed a horse in the future, you are responsible to know the above information and combination results - the registries' safeguards certainly have the best of intentions, but they simply do not guarantee the horse you choose to breed to or purchase won't end up being ASD-positive. I suggest to be safe you choose to breed clean mares and stallions together, for there are far more clean horses than ASD/carrier horses and finding suitable breeding stock won't be difficult at all.