Don't miss out on up to 30% off our most popular tests!

Shop Now

Muscular Hypertrophy (Double Muscling)

Muscular Hypertrophy (Double Muscling) is a condition where the muscles of the body become much larger and more pronounced than normal, due to a defect in a gene that helps to regulate muscle growth.

Key Signs

Increased muscle mass

Age of Onset

At birth

Present at birth


Autosomal Recessive

For autosomal recessive disorders, dogs with two copies of the variant are at risk of developing the condition. Dogs with one copy of the variant are considered carriers and are usually not at risk of developing the disorder. However, carriers of some complex variants grouped in this category may be associated with a low risk of developing the disorder. Individuals with one or two copies may pass the disorder-associated variant to their puppies if bred.

Likelihood of the Condition

High likelihood

At risk dogs are highly likely to show signs of this disease in their lifetime.

What to Do

Here’s how to care for a dog with Muscular Hypertrophy

Partner with your veterinarian to make a plan regarding your dog’s well-being, including any insights provided through genetic testing. If your pet is at risk or is showing signs of this disorder, then the first step is to speak with your veterinarian.

For Veterinarians

Here’s what a vet needs to know about Muscular Hypertrophy

Dogs that are homozygous for the mutation are highly over-muscled. Heavily muscled Whippets, also called "Bully Whippets", have broad chests and unusually well-developed leg and neck musculature. Bully Whippets can easily be distinguished from their normal littermates based on physical appearance. Double muscled Whippets don't seem to have any health problems other than occasional muscle cramping.

Treatment is supportive should an affected dog suffer from muscle cramping.

For Breeders

Planning to breed a dog with this genetic variant?

There are many responsibilities to consider when breeding dogs. Regardless of test results it is important that your dog is in good general health and that you are in a position to care for the puppies if new responsible owners are not found. For first time or novice breeders, advice can be found at most kennel club websites.

Breeding for the double muscle appearance is not advised as dogs that are over-muscled may have impaired movement. A carrier dog with one copy of the Muscular Hypertrophy mutation can be safely bred with a clear dog with no copies of the Muscular Hypertrophy mutation. About half of the puppies will have one copy (carriers) and half will have no copies of the Muscular Hypertrophy mutation. Puppies in a litter which is expected to contain carriers should be tested prior to breeding. Carrier to carrier matings are not recommended for this condition as the resulting litter may contain dogs with the double muscle phenotype. Please note: It is possible that signs similar to the ones caused by the Muscular Hypertrophy mutation could develop due to a different genetic or clinical cause.

Technical Details

Variant T>A
Chromosome 37
Coordinate Start 729,361
Coordinate End 729,360

All coordinates reference CanFam3.1

References & Credit

Credit to our scientific colleagues:

Mosher, D. S., Quignon, P., Bustamante, C. D., Sutter, N. B., Mellersh, C. S., Parker, H. G., & Ostrander, E. A. (2007). A mutation in the myostatin gene increases muscle mass and enhances racing performance in heterozygote dogs. PLoS Genetics, 3(5), 779–786. View the article