What is a Double Merle?

“Merle” is a gorgeous variation in coat color that results in dappled fur, often white with gray & black, or white with brown & tan. Merle is an acceptable standard color in breeds such as Australian Shepherds, Collies, Dachshunds, Great Danes, and many more. According to the American Kennel Club, “The merle coat color lies in basic genetics, where there is a dominant and recessive trait to produce those gorgeous splotches.”

A regular merle dog carries one dominant (M) and one recessive (m) merle allele. When two merles are bred together (Mm x Mm), then each resulting puppy has a 50% chance of being a regular merle (Mm), a 25% chance of being a non-merle (mm), and a 25% chance of inheriting both dominant merle genes (MM). When both dominant genes are present, we call this a “double merle,” and they can be born with varying degrees of vision and hearing issues. They may be hard-of-hearing, deaf in one or both ears, have mild to serious eye defects, and may even be missing eyeballs entirely.

This is because the pigment cells affected by the merle gene are produced by the same embryonic tissue as the skin, bones, heart, eyes, and ears. When two copies of the merle gene cause an extreme reduction of pigmentation, this can lead to abnormal cell development within the inner ears and various structures within the eyes. There also seems to be a correlation between double merles and certain heart issues, such as persistent right aortic arch.

Benny, the deaf, double merle Great Dane

The easiest way to avoid producing a double merle puppy is to never breed two merle dogs together. Breeding a merle with a non-merle gives the same percentage of likely merles (50%) as breeding two merles together, without the dangerous probability of producing a double merle. This is why no ethical breeder would ever take the risk of breeding two merles together.

The prevalence of double merles is due to negligent, unethical breeding done by backyard breeders and puppy mills. In fact, one of Nashville Canine Coaching’s resident rescue dogs is a double merle “Mini Aussie” who rescued during a crackdown on an Amish puppy mill. In order to stop this epidemic of unethically-bred dogs, we all need to stop supporting puppy mills: never purchase a puppy from a pet store or online; only purchase a puppy from an ethical breeder who can show you their animals’ medical records, allow you to meet the dogs, and see where the puppies are raised; or best of all, adopt instead of shop!

Smorty, the rescued double merle Aussie, with Nashville Canine Coaching owner, Ashley.