Khov after reading this I did a little looking around and Satori is right, whippets with a single mutated gene are desired because they are extremely fast, if they have two mutated genes they become double muscled or "bully whippets" problem is that the heart and lungs do not increse in size so they are unable to provide enough oxygen to feed the muscles which leads to muscle cramps from excersise (since muscles cannot recover at a normal rate) as well as exceedingly short life expectancies due to the added stress on the heart.
Correct, it's from the gene doubling, but the problem with Bully Whippets is you can't treat them like a normal whippet. You have to treat a B.W. like you would treat a larger dog like a St. Bernard.
ie, don't take them to the speed or agility courses, don't play endless frisby with them, don't sprint with them, etc. All stuff that's great for normal whippets, but aren't ideal for the B.W.'s frame.