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Discovering the Double Merle: A Deep Dive into Canine Genetics and Double Merle Breeding

A red and white Bernedoodle sitting in the snow.
A Bernedoodle with a heavy white pattern.

When I first chose Coco, my Bernedoodle, it was her sweet personality and unique coat—a charming mix of gray, black, white, and tan—that caught my eye. I later learned that this coloration, known as merle, is quite rare and I wanted to learn more about it.

Understanding Merle coats

The merle pattern is a visually captivating genetic marvel, causing a mottled intermingling of colors on a dog's coat. It's a result of a specific gene influencing pigmentation, leading to diluted patches that coexist with the normal base color. While the merle pattern contributes to a dog's striking appearance, it can come with significant health concerns if not managed carefully.

Two Bernedoodles sitting and staring up.
Coco and Kira with their cool merle coats.

Health Implications: A Cautionary Tale

The notion of breeding merle dogs comes with a caveat: double merle breeding can lead to offspring with serious health issues, including blindness and deafness. This sparked my quest for a deeper understanding of merle genetics to ensure responsible breeding.

The Science Behind Merle Genetics

The merle pattern arises from a mutation in the M gene. It's an incomplete dominant trait, meaning only one copy of the mutated gene is needed for the pattern to manifest. Dogs with one merle gene (Mm) will display the pattern, but those with two (MM) can have extensive white areas and are at risk of health complications.

Merle patterns vary significantly, and the expression of this gene doesn't lead to a uniform look—it can result in anything from large blotches to a speckled effect. When two merles breed, there's a 25% chance for each puppy to inherit the double merle genotype (MM), which can lead to sensory impairments.

My DNA Discovery: Expectation vs. Reality

I had assumed both my dogs were heterozygous merles (Mm), ideal for breeding without health risks. Coco's DNA confirmed this, but Kira's test revealed an MM genotype—a double merle. This was puzzling, as she exhibited a darker coat with minimal white and no apparent health issues.

The Cryptic Merle Explained

Further research led me to resources explaining the complex nature of the merle gene. It turns out, merle expression is influenced by the length of a DNA insertion. A "cryptic" merle, with an insertion length between 200-230, can lead to minimal coat changes and no adverse health effects. This likely explains Kira's condition—she's healthy despite being a double merle, thanks to one allele being longer and the other "cryptic."

Safe Breeding Practices With Merle Dogs

To breed merle dogs responsibly, it's crucial to ensure the other parent is non-merle (mm), verified through DNA testing. This approach prevents the creation of double merle (MM) puppies, safeguarding against the associated health issues.

A Tri-Color Bernedoodle next to a Merle spotted Bernedoodle
Good: mm and Mm will produce mm and Mm puppies

Two Merle Bernedoodles with black spots.
Bad: Mm and Mm will produce MM, Mm, and mm puppies

A Tri-Color Bernedoodle next to a white Bernedoodle
Breeding mm with MM will get us back safely to Mm

Commitment to Canine Health at Oodles of Bernedoodles

At Oodles of Bernedoodles, the health and well-being of our dogs are paramount. DNA testing is an essential tool in our commitment to nurturing a healthy new generation of Bernedoodles, free from preventable genetic conditions.

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