In whippets, a type of racing dog, a single copy of a mutated gene for myostatin helps them be the fastest sprinters on the track, report researchers led by Elaine Ostrander from the National Human Genome Research Institute. But having two copies of this MSTN fast-twitch muscle gene doesn't make them doubly fast. Instead, those dogs are bulky with muscle and breeders usually put them down shortly after birth.
Good thing Snoopy was a beagle, and not that fast.