Last year I got a female golden retriever puppy that was considered the runt of the litter, but multiple vets declared her healthy. I was convinced there was something wrong with her and insisted they check for various congenital defects, but none were found. She has maxed out at 45 lb (parents were around 70 lb) and always had a low interest in food but a high activity level and apparently good health. At about 10 months, she showed signs of proestrus, but did not progress through a normal estrous cycle. Based on her two main symptoms -- stunted growth and sterility, I had her karyotyped by Texas A&M and found her to be missing an X chromosome. There is very little information about Turner's syndrome (monosomy X) in dogs, although it is occasionally found in horses and they are reported to be small and sterile but otherwise healthy. Personally, I suspect that this may be more common than believed in canines -- just that almost no one tests for it.