Each puppy within a merle-to-merle litter has a 25 percent chance of being born "double merle," Christina Lee of Deaf Dogs Rock told The Dodo. Being born "double merle" can lead to the afflictions mentioned above. "Some [canine] deafness can be caused by bad breeding," said Lee. "For example, merle-to-merle breeding ... has a 25 percent chance of causing deafness or vision impairment in each puppy born in a litter from [improper] breeding."
Some breeders don't take the health and welfare of the puppies into account, which can cause puppies who are bred improperly to become deaf or blind. What's worse, some breeders are instructed to euthanize a dog when he is born deaf, according to Lee. "We like to educate breeders that Deaf Dogs Rock can list [these dogs] as available for adoption or we can sponsor the deaf puppies into special needs rescues we work with across the U.S," Lee said.
Breeds with a higher rate of deafness due to improper breeding include Australian shepherds, pit bulls, Great Danes, Dalmatians and American bulldogs.