I'm late getting around to looking at this topic, but now that I have read through it, I have to comment.
Do some people seriously believe that a limited gene pool if AMHA closes is anything like a successful line breeding program? I'm afraid I do not see how one of those has anything to do with the other. I know this has already been said but I will repeat it, just to keep my thoughts all in order. Yes, line breeding works, IF you do it right and if you choose the right individuals to base your line breeding program on. The thing is, for line breeding to be successful you must select your breeding stock for a specific reason—you do NOT just take whatever happens to be available and start linebreeding (or at least you shouldn't do that). Someone pointed out to me today that AMHA could/should take a lesson from the Friesian breed--the Friesians have had a very small gene pool, and that gene pool does include dwarfism....with the breeding going on within that breed, dwarfism is now a huge problem for the breed. Eliminate dwarf carriers and they won't have many horses left. You can get a lot of info on the issue through a search on google. This exact same thing can happen in AMHA if it closes completely. Some people believe that all Miniature horses carry the dwarf gene now; I personally do not believe this is the case. If it were, we would see a lot more dwarf foals born than there actually are. However, a closed registry could very possibly lead to that very thing coming true in the not so distant future.
Dwarfism in the Shetlands…perhaps at one time it did exist. I cannot say for sure that it did or didn't; I have heard more than one person say that they did see, sometime a long time ago, a Shetland pony that was a dwarf. So, I have to assume that it is entirely possible that there were a few dwarf Shetlands. However--I do believe people who say that now the ASPC Shetlands have no dwarfism. Think about it—what self respecting ASPC breeder wants a dwarf (or a dwarf producer) in his herd? A dwarf is the complete opposite of everything that a Shetland breeder is striving for. Pony breeders want long legs, slim bodies, long necks and above all—good movement. A dwarf has none of those things. It makes perfect sense that they would cull dwarfism from their herds, and there is no reason why they would not be successful in eliminating dwarfism from the breed. The majority of Mini breeders do not seem interested in eliminating dwarfism, they simply cover it up by breeding their dwarf producers to different horses in hopes that the gene won't make an appearance again. Start line breeding with some dwarf carriers and the gene will most certainly reappear--with increasing frequency.
I will stop there, though there is more I would like to say. maybe another time.