This discussion is now getting to the nitty-gritty of the dwarfism issues. This is likely the reason that the letters I wrote to MHW (which didn't publish it) and the Journal (which did but did not seem to get any reaction) got the reactions they did (or didn't!) get. It would have been nice to see some of this type of discussion in the breed magazines. I think it is hard to make decisions without knowing how many horses will test positive for one of the dwarf genes, but I believe John Eberth estimates it may be 25-40%. One perhaps unintended consequence of the recent re-infusion of ASPC blood into minis will be the reduction in those numbers.
I have hair samples from one mare on the way to be tested, but since I am no longer breeding, it really doesn't matter to me how she tests. I have absolutely no reason to think she will be positive for any of the 4 genes, but we will see. She is for sale, and my plan is to post the test results in the ads. I want to set an example. I would like to see dwarfism treated like LWO, which I am quite familiar with. As long as horses that are carriers of the gene suffer no consequences (until they are crossed with another carrier) I see no reason to eliminate them from the gene pool (like LWO). Dwarfism is more complex because 4 genes are involved, but (as I understand it) as long as you don't cross certain genes, you will be OK. Certainly if you cross a horse negative for all 4 genes with one positive for one or more you will not get a dwarf, but maybe another carrier. But dwarfism is also different from LWO in that some of the combinations produce early abortions and NOT LIVE DWARFS. Like those mares that "slip a foal" or are hard to get in foal. From John's thesis, it looks like about half the gene combinations that produce "dwarfs" don't produce live dwarfs, but an early fetal loss. How many breeders have had one of those? We had one several years ago where the mare was U/S as pregnant but never foaled. Was she a carrier? I have no idea and we no longer own her. Most breeder probably just breed the mare again, and don't even think about the fact it might have been a dwarf.
I see the testing as making it much easier to figure out which horses to cross safely with no chance of producing a live dwarf. And that breeders may not all use the information the same way - some might risk a slipped foal if a live dwarf is not possible from the gene combinations. Some breeders already know which stallions and mares are carriers so maybe they feel they don't need to test. But I think having information on which genes are involved will be useful to breeders who understand the genetics.