Just to clarify
Belgian Blues and whippets are the only two animals that have this mutation in the regular gene pool...
Myostatin mutations have been found in humans:
https://www.nejm.org/doi/full/10.1056/NEJMoa040933
It was the previous work on the myostatin knockout mouse that lead the doctors to investigate if the muscle baby had a mutation in the myostatin gene: http://www.pnas.org/content/98/16/9306
That's true, but we don't really have it commonly in our gene pool. Maybe it will happen when this baby grows up and starts reproducing, but as of right now there's no need to worry about having this mutation.