There is a gene mutation found in some Whippets that creates a dramatic effect known as double muscling and results in a "Bully" Whippet. Technically speaking it is a 2-bp deletion in the MSTN gene.In the homozygote
state this results in the double-muscling phenotype known as
the "Bully" Whippet. The mutation creates a premature truncation of the
protein at amino acid 313, removing about 17% of the protein. The only other animals this has been found in appears to be cattle and sheep.
There is a great article that gets into the specifics of the gene mutation and how it appears to happen in the Whippet gene pool. It is called A Mutation in the Myostatin Gene Increases Muscle Mass and Enhances Racing Performance in Heterozygote Dogs Please click the title to be brought to the article at PubMed Central.
In 2007 The New York Times published a very informative article that discussed how breeders were employing eugenics in their breeding programs and the potential problems that come into play with such pointed gene selection.The article was entitled As Breeders Test DNA, Dogs Become Guinea Pigs.
In these photos of Wendy, taken by Stuart Isett for The New York Times, you see an excellent example of how the mutation effects the musculature of a normally slender Whippet.
An interesting chart that explains the spread of the mutation was
featured in the NY Times article and is included below.