What is the Double Merle Gene?

When we adopted Penny from a rescue specializing in dogs with the double merle gene, we knew very little about the condition. We were considering adopting a deaf dog since we already use ASL at home. Little did I know about the connection between deafness and this genetic condition.

Despite working in the dog world for over a decade, I was surprised at how little I knew about the double merle gene. So, I dove into the research. Here’s what I’ve learned.

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What Does “Merle” Mean?

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

The gene responsible for the merle pattern is the same across all dogs, indicating it predates dog breeds.

What is a Double Merle?

A double merle occurs when a dog has two copies of the merle gene. Unlike single merle dogs, double merles, like Penny, are usually mostly or entirely white with smaller pigment splotches. This loss of pigment is linked to many hereditary disorders.

Why Can’t Two Merles Breed?

A double merle arises when two merles are bred together, giving each puppy a 25% chance of being blind, deaf, or both. Penny is bilaterally deaf and blind in her right eye, presenting inherent safety challenges we are working through.

Can a Double Merle be Healthy?

Yes, it’s possible for a double merle to be healthy. However, breeding double merles is irresponsible because many puppies will be born deaf, blind, or both—conditions that are entirely preventable. According to a study on the merle gene, 15% of double merles are bilaterally deaf, and 10% are unilaterally deaf.

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

If your dog has mostly white fur with some patches, they might be a double merle. Genetic tests can confirm this. Since we adopted Penny from a rescue specializing in double merle dogs, we knew her genetic background.

Are All Double Merle Dogs Blind?

Not all double merle dogs are blind, but they can experience various vision issues. Penny’s left eye has a jagged pupil, and her right eye has microphthalmia, meaning it is small and likely non-functional. Other common conditions include corectopia and cataracts. For more on these conditions, check out this post from Eye Care for Animals.

Are Double Merle Dogs Aggressive?

Contrary to some misconceptions, double merle dogs are no more aggressive than other dogs. Penny, for example, is the sweetest, gentlest dog we’ve ever had. Despite their disabilities, double merles are not inherently more prone to aggression or behavior issues.

What Health Issues Do Double Merles Have?

Double merle dogs can suffer from sunburns and skin cancer due to their lack of pigment. Pet-friendly sunscreen is essential. Additionally, a study found that sensory-impaired dogs, including double merles, do not exhibit more health or behavioral problems than typical dogs, though they are more prone to OCD behaviors.

For a deeper dive into the study on sensory-impaired dogs, you can read it here.

In summary, while double merle dogs like Penny can face unique challenges, with the right care and understanding, they can lead healthy, happy lives.