WELCOME TO MFK!!!
TO my knowledge they have they disk cut when still young.
It is a possibility that inbreeding will cause that. Most genetic flaws will not show up in the wild. When you have both parents having the same genetic flaw, inbreeding will isolate the certain chromosomes. That will have a better chance of creating an animal with a certain feature. The same process works on most animals. The short bodied fishes that you see out there didn't sudden became a strain. Someone created it.I always thought it was a genetic disorged caused by excessive inbreeding. I thought it was similar to a cleft pallet