FORT MOTT STATE PARK, N.J. — When mutant, muscle-bound puppies started showing up in litters of champion racing whippets, the breeders of the normally sleek dogs invited scientists to take DNA samples at race meets here and across the country. They hoped to find a genetic cause for the condition and a way to purge it from the breed.

It worked. “Bully whippets,” as the heavyset dogs are known, turn out to have a genetic mutation that enhances muscle development. And breeders may not want to eliminate the “bully” gene after all. The scientists found that the same mutation that pumps up some whippets makes others among the fastest dogs on the track.

With a DNA screening test on the way, “We’re going to keep the speed and lose the bullies,” Helena James, a whippet breeder in Vancouver, British Columbia, said.

Free of most of the ethical concerns — and practical difficulties — associated with the practice of eugenics in humans, dog breeders are seizing on new genetic research to exert dominion over the canine gene pool. Companies with names like Vetgen and Healthgene have begun offering dozens of DNA tests to tailor the way dogs look, improve their health and, perhaps soon, enhance their athletic performance.