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 the condition ourselves. We were exploring the idea of adopting a deaf dog since we already use ASL in our home. Little did I know the connection between deafness and this genetic condition.

As someone who has worked in the dog world for more than a decade, I wondered how I knew so little about the double mere gene. So, of course, I dove into the research. Today, I’m sharing what I’ve learned.

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First, what does “merle” even mean?

In order for there to be a double merle, there needs to be a merle, right?

Here’s the official definition from the American Kennel Club: “Also known as dapple, merle 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 in every dog–which is fascinating because that means the gene arose before dog breeds.

What is a double merle?

If a dog has two copies of the merle gene, it’s called double merle. Unlike their merle counterparts, their coats don’t have that dappled appearance. In fact, these pups, like Penny, are usually mostly or entirely white. Pigment splotches are smaller than their single merle relatives.

The loss of pigment is thought to be responsible for many of the hereditary disorders we’re going to touch on.

Why can’t two merles breed?

A double merle arises when two merles are bred together. When two merles are bred together, each puppy has a 25 percent chance of being blind, deaf, or both. There’s a really great image on this post that explains the DNA makeup of this breeding pair.

Penny is bilaterally deaf (which means deaf in both ears) and blind in her right eye (more on her vision challenges below). Yes, we are training together, but there are a lot of inherent safety issues with being deaf and partially sighted we need to work through. (I’m thinking about getting something like this for her as an alert to others.)

Can a double merle be healthy?

Sure.

Of course it’s possible.

And, also, it’s irresponsible to breed double merles knowing that many of the dogs will be born deaf, blind, or both, because it’s entirely preventable.

In the data, a study on deafness and the merle gene found: “For single merles (Mm), 2.7% were unilaterally deaf and 0.9% were bilaterally deaf. For double merles (MM), 10% were unilaterally deaf and 15% were bilaterally deaf.” So, not even most. But lots.

How do I know if my dog is double merle?

If your dog has mostly white-colored fur and some patches, he or she might be a double merle. Genetic tests are available, too.

In our case, since we adopted Penny from a rescue that specializes in double merle dogs, we knew.

Are all double merle dogs blind?

Not all double merle dogs are blind, and blindness isn’t the only vision condition they can experience.

In addition to blindness, double merles can have jagged pupils that make them sensitive to light. Penny’s left eye has a jagged pupil

Microphthalmia, which means small eye, is common in these dogs, too. The eye can seem almost normal or not present at all–it’s a huge range. This is Penny’s right eye, and she doesn’t seem to have any vision in this eye.

Other conditions include corectopia and cataracts.

For more on these conditions and how the gene affects vision, check out this post from Eye Care for Animals.

Are double merle dogs aggressive?

I can’t begin to imagine where this misconception came from, but–in short–no. Despite their disabilities, double merle dogs are no more or less aggressive than the general population of dogs. And, in our small sample size, Penny is the sweetest, gentlest, most open dog we’ve ever had!

Regardless of where the assumption of aggression came from, it’s apparently paired with other health- and behavior-related assumptions. Which brings me to:

What health issues do double merles have?

First, double merle dogs can suffer from sunburns and skin cancer. If you’re the person of one of these dogs, talk to your vet about a pet-friendly sunscreen. In fact, at her very first checkup, our vet made me promise to always keep Penny’s nose sunscreened!

What else?

In a fascinating study, “Are dogs with congenital hearing and/or vision impairments so different from sensory normal dogs? A survey of demographics, morphology, health, behaviour, communication, and activities,” the authors tackled assumptions about dogs with genetically-related sensory impairments. This included double merle dogs, as well as others, plus a typical dog group.

The authors note that the number of dogs with impairments is increasing but that their future is bleak. They write:

These pessimistic scenarios result from popular assumptions predicting that dogs with congenital hearing/vision impairments exhibit severe Merle-related health troubles (cardiac, skeletal, neurological), impairment-related behavioural troubles (aggressiveness, anxiety), and poor capacities to communicate, to be trained, and to be engaged in leisure or work activities.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7473589/

So, they conducted a massive global survey to test those assumptions. What did they find?

Contrary to the common assumption [5], the sensory impaired cohort, including 131 “presumed” double Merles, did not exhibit more heart, skeletal, skin, digestive or other health troubles than the sensory normal cohort. … Contrary to another popular assumption, but in agreement with a past study [22], sensory impaired dogs did not exhibit more aggressiveness, anxiety, ADHD or other behavioural troubles than sensory normal dogs. However, OCDs, that can mimic neurological signs, were much more frequently reported in sensory impaired dogs.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7473589/

In other words, despite assumptions of both behavioral and health problems among these pups, the data doesn’t show that to be the case.

BTW, if you’re the person of one of these dogs, I highly recommend a full read of the study. It’s fascinating.

What is the life expectancy of a double merle?

Despite all of the hereditary conditions they might experience, the life expectancy of a double merle dog is similar to his or her single merle relatives. These dogs can be relatively healthy and can live a regular life.

So, why are there still double merle dogs?

Even though these dogs can breed, it’s considered unethical because of all the associated risks. So, why does it still happen?

Well, in lots of cases, they’re accidental.

Penny came from a hoarding situation where, presumably, the person did not carefully select her dogs for breeding.

These types of backyard breeding, accidental situations continue to arise–as noted in the increase in sensory disabilities in dogs in the study cited above.

It would take an enormous investment in animal welfare to curtail the breeding of double merle dogs, which is why this persists.

Double Merle Dogs Can Make Wonderful Family Pets

Like Penny, who is loved and beloved in our home!

We are so lucky to be her family. (Here’s how she’s doing, one month in!)

I’m in several Facebook groups for double merle dogs and deaf dogs, and the consensus is these dogs are absolutely wonderful family pets.

Follow Penny’s journey from caged at a hoarder’s to living the good life on Instagram!

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