A Bully Whippet - Above, this is Wendy. She has two copies of a double muscling phenotype.
A racing whippetWhippets are sometimes used in racing and breeders of these racing whippets found that some successful racing dogs with increased muscling produced progeny with this muscling trait and similar racing success. However, if two carriers of this trait were bred together, these dogs sometimes produced heavily muscled dogs that gave the whippets a "bully" appearance such as that which Wendy has in the top photo.
Chart I found on The New York Times site. See the whole article at the link at the bottom of this post-Breeders got together and tested both affected and unaffected dogs to try and find the gene causing these distinctions. It has been found that the wild-type whippet in this case meaning the normal whippet, did not have a copy of this mutation that affects muscling. Intermediate affected dogs, successful racers with more muscling than average, had one copy of this mutated gene. Bully whippets all had a double copy of the gene.
Author Summary
An individual's genetic profile can play a role in defining their natural skills and talents. The canine species presents an excellent system in which to find such associative genes. The purebred dog has a long history of selective breeding, which has produced specific breeds of extraordinary strength, intelligence, and speed. We have discovered a mutation in the canine myostatin gene, a negative regulator of muscle mass, which affects muscle composition, and hence racing speed, in whippets. Dogs that possess a single copy of this mutation are more muscled than normal and are among the fastest dogs in competitive racing events. However, dogs with two copies of the same mutation are grossly overmuscled, superficially resembling double-muscled cattle known to possess similar mutations. This result is the first to quantitatively link a mutation in the myostatin gene to athletic performance. Further, it emphasizes what is sure to be a growing area of research for performance-enhancing polymorphisms in competitive athletics. Future implications include screening for myostatin mutations among elite athletes. However, as little is known about the health issues and potential risks associated with being a myostatin-mutation carrier, research in this arena should proceed with extreme caution.
Some info from the study below...
A Mutation in the Myostatin Gene Increases Muscle Mass and Enhances Racing Performance in Heterozygote Dogs Dana S. Mosher, Pascale Quignon, Carlos D. Bustamante, Nathan B. Sutter, Cathryn S. Mellersh, Heidi G. Parker, Elaine A. Ostrander
National Human Genome Research Institute, National Institutes of Health, Bethesda, Maryland, United States of America, Department of Biological Statistics and Computational Biology, Cornell University, Ithaca, New York, United States of America, Animal Health Trust, Center for Preventive Medicine, Newmarket, United Kingdom
Double muscling is a trait previously described in several mammalian species including cattle and sheep and is caused by mutations in the myostatin (MSTN) gene (previously referred to as GDF8). Here we describe a new mutation in MSTN found in the whippet dog breed that results in a double-muscled phenotype known as the “bully” whippet. Individuals with this phenotype carry two copies of a two-base-pair deletion in the third exon of MSTN leading to a premature stop codon at amino acid 313. Individuals carrying only one copy of the mutation are, on average, more muscular than wild-type individuals (p = 7.43 × 10−6; Kruskal-Wallis Test) and are significantly faster than individuals carrying the wild-type genotype in competitive racing events (Kendall's nonparametric measure, τ = 0.3619; p ≈ 0.00028). These results highlight the utility of performance-enhancing polymorphisms, marking the first time a mutation in MSTN has been quantitatively linked to increased athletic performance.
Introduction
The wide variety of behaviors and morphological types exhibited among dog breeds and the overall low genetic diversity within each breed make the dog an excellent genetic system for mapping traits of interest. Recently, owners of whippets, an established racing-dog breed, have reported a phenotype of heavy muscling occurring within the breed (http://www.k9community.co.uk/forums). The typical whippet is similar in conformation to the greyhound, a medium-sized sighthound, weighing about 9 kg and characterized by a slim build, long neck, small head, and pointed snout. 
Heavily muscled dogs, termed “bully” whippets by breeders, have broad chests and unusually well-developed leg and neck musculature. “Bully” whippets are easily distinguished from their normal littermates based on physical appearance alone. Owners report that “bully” whippets do not have any health abnormalities other than muscle cramping in the shoulder and thigh. However, the dogs are often euthanized at an early age as they do not conform to the American Kennel Club breed standard. In addition, about 50% of “bully” whippets have a distinctive overbite. [see pic on right]
To see the paper on this study, go here.
Normal whippet - "wild type"
Intermediate whippet - One copy of the mutation
Bully whippet - Two copies of the mutation, 50% have overshot muzzleIt is interesting that half of the Bully Whippets have the upper mandible so long as it is.
Related article:
As Breeders Test DNA, Dogs Become Guinea Pigs - New York TimesLabels: genetics
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