I don't mean to try and speak for the other posters, but personally I have never believed that miniatures were a pure offshoot from Shetlands. That just never made sense, yes there is a lot of Shetland blood in the miniatures as we know them today. But any small horse they could find was used to produce the miniatures of yesterday. Including fabellas, English Shetlands, Morgans, pit ponies of different heritage etc etc.
What I am saying though is that in the 1,000's of samples John Eberth is working with, he personally told me he had not found any incidents of dwarfism in the Shetland samples he has worked with.
To me that does not mean there is none there as that would not be logical. I personally think that it is there, but over the years it has been diluted more and more with each new generation of Shetlands as they have moved away from the really small horses and towards what they have bred for in that breed. It is his opinion also that the registry needs to be left open to be able to input more blood into our modern miniatures.
This of course is a controversial topic for many miniature people, which I understand completely. Rationalization and denying that the problem is there will only magnify the problem in years to come from my standpoint.
With that said though, the incidence of dwarfism is reducing as we are for the most part breeding for leggier and more refined miniatures now days.
The natural selection process caused by the show ring results, peoples preference etc. is doing a good job and things are changing for sure.
Just look at the horses of 15 years ago and what you see now days in the show ring. There sure is a big difference. Todays breeders are doing a great job in my opinion.