Bulldogs World

The merle gene is often associated with congenital deafness, with merle dogs being more likely than other dogs to be born deaf. Dogs with two copies of the merle gene (homozygous merle or "double merle") have an even higher chance of being born deaf.[13] The UK Kennel Club has acknowledged the health risk associated with homozygous merle and will stop registering puppies produced from merle to merle matings starting from 2013. Merle to merle mating is currently only forbidden in three breeds.[14] Recent research indicates that the majority of health issues occur in dogs carrying both piebald and Merle genes. The piebald gene is indicated by white areas on the dog's coat as seen in the red merle Catahoula pictured below.

The suppression of pigment cells (melanocytes) in the iris and in the stria vascularis of the cochlea (inner ear) leads to blue eyes and deafness. An auditory-pigmentation disorder in humans, Waardenberg syndrome, reflects some of the problems associated with heterozygous and homozygous merle dogs and genetic research in dogs has been undertaken with the goal of better understanding the genetic basis of this human condition.[10]