What is the Double Merle Gene?

When we adopted Penny from a rescue that focuses on dogs with the double merle gene, we knew very little about this condition. We were considering adopting a deaf dog since we already use ASL in our home, but I didn’t initially realize the connection between deafness and the double merle gene.

Having worked in the dog world for over a decade, I was surprised by how little I knew about the double merle gene. Naturally, I dove into research, and today I’m sharing what I’ve learned.

What Does “Merle” Mean?

To understand double merle, we first need to know what merle is. According to the American Kennel Club, merle, also known as dapple, is characterized by irregular blotches of fur on a lighter background of the same pigment, such as solid black on gray (blue merle) or solid brown on tan (red merle). Blue and partially blue eyes are often seen with the merle pattern.

The merle pattern gene is the same in every dog, indicating that this gene arose before the formation of distinct dog breeds.

What is a Double Merle?

A dog with two copies of the merle gene is known as a double merle. Unlike their merle counterparts, double merle dogs typically have mostly or entirely white coats with smaller pigment splotches. This loss of pigment is linked to many hereditary disorders.

Why Can’t Two Merles Breed?

When two merles breed, each puppy has a 25% chance of being blind, deaf, or both. For example, Penny is bilaterally deaf and blind in her right eye. Training and ensuring her safety are challenging due to these sensory impairments.

Can a Double Merle Be Healthy?

While it is possible for a double merle to be healthy, breeding them is considered irresponsible because many of these dogs will be born deaf, blind, or both. Studies have shown that single merles (Mm) have a 2.7% chance of being unilaterally deaf and 0.9% chance of being bilaterally deaf, whereas double merles (MM) have a 10% chance of being unilaterally deaf and a 15% chance of being bilaterally deaf.

How Do I Know if My Dog is a Double Merle?

If your dog has mostly white fur with some patches, it might be a double merle. Genetic tests are available to confirm this. We knew Penny was a double merle since we adopted her from a rescue specializing in these dogs.

Are All Double Merle Dogs Blind?

Not all double merle dogs are blind. However, they can experience various vision issues, such as jagged pupils that make them sensitive to light or microphthalmia (small eye), where the eye may seem almost normal or not present at all. Penny, for example, has a jagged pupil in her left eye and is blind in her right eye.

Are Double Merle Dogs Aggressive?

Despite some misconceptions, double merle dogs are no more aggressive than the general dog population. In fact, Penny is the sweetest, gentlest dog we’ve ever had.

What Health Issues Do Double Merles Have?

Double merle dogs can suffer from sunburns and skin cancer due to their lack of pigment. Our vet emphasized the importance of using pet-friendly sunscreen on Penny’s nose.

A study titled “Are dogs with congenital hearing and/or vision impairments so different from sensory normal dogs?” examined various assumptions about these dogs, including those with the double merle gene. The study found that sensory-impaired dogs did not exhibit more health or behavioral issues than sensory-normal dogs, except for a higher incidence of OCDs.

Contrary to common assumptions, double merle dogs do not have more cardiac, skeletal, skin, digestive, or other health troubles. They also do not exhibit more aggressiveness, anxiety, ADHD, or other behavioral issues compared to sensory-normal dogs.

If you have a double merle dog, I highly recommend reading the full study. It provides a detailed and fascinating insight into the lives of these unique dogs.