Double Merles

Aston was born blind and deaf because he is a double merle. A dog with a merle allele (a variation of a gene) typically has a marbled coat pattern with patches of diluted fur. A double merle has so much merle that some pigmentation is deleted to white, which causes risk of hearing and vision impairments.

What is Merle?

Merle and non-merle are two variations of a gene; every dog inherits one or the other from each parent. The merle allele is characterized by dilution of a dog's fur, skin, and eyes, so a dog with a merle allele often has a marbled coat pattern and blue eyes. However, any dog with a merle allele is merle, regardless of their appearance.

Merle Can be Hidden

Merle primarily impacts black and brown pigmentation (eumelanin) and has a harder time diluting red, yellow, and cream pigmentation (phaemelanin), As a result, merle is often hidden in recessive red (e/e) dogs like Aston and his dad because they produce only phaemelanin in their fur. Merle can also be hidden in clear sable dogs for the same reason.

What is a Double Merle?

Since merle is incomplete dominant, two merle alleles impact a dog differently than just one merle allele. A double merle inherits a merle allele from both parents and has some pigmentation deleted to white instead of diluted to a lighter color. This impacts more than just their appearance and also risks hearing and vision impairments.

More specifically, lack of pigmentation in a dog’s inner ear can cause the nerve fibers to die weeks after birth, resulting in deafness. Lack of pigmentation in the eyes during fetal development can result in eye deformities and blindness. The level of hearing and vision impairments (if any at all) varies for each double merle.

The Impact of Merle Allele Length

The length of a merle allele impacts its physical expression and risk level when combined with other alleles. As illustrated by Corinne Benavides' chart based on the merle research by Langevin et al (2018), there is no risk associated with combining some of the shorter allele lengths, but the risk increases as longer lengths are combined.

The cryptic merle (Mc) allele is so short that it appears and breeds the same as a non-merle (lowercase "m") allele. Other lengths can also be non-expressing, but only the cryptic merle allele can be safely combined with any other merle allele without any risks.

The harlequin merle (Mh) allele, which is different from harlequin found in Great Danes, is so long that it has low risk of hearing impairments without a second merle allele. Many harlequin merle dogs have a lot of diluted pigment and are very visibly merle, but some express as minimally merle or are occasionally even non-expressing.

White dog with black/blue merle on head, ears, and back

Aston's mom is visibly merle.

Aston's dad is a recessive red hidden merle.

Aston is a recessive red double merle.

Aston's brother Martin looks like a more stereotypical double merle since he is not recessive red like Aston.

Five newborn puppies, including two deaf/blind double merle puppies, a deaf/partially blind puppy, and two unimpaired solid brown puppies

Aston and his littermates.

How Merle Impacts Aston and his Family

Aston’s parents were not genetically tested, so the following assumptions about their genetics are based on their physical appearance and Aston’s DNA test results.

Aston's mom is visibly merle with partially blue eyes and a lot of white fur. She has a harlequin merle (Mh) allele and carries a recessive red allele (e). Aston's dad is mostly cream with small amounts of white fur. He also has a harlequin merle (Mh) allele, but his merle is hidden because he has two recessive red alleles (e/e).

Aston inherited a harlequin merle allele from each parent (Mh/Mh), which is the highest risk combination of two merle alleles. He is a cream and white double merle who was born fully blind and deaf. Aston also inherited a recessive red allele from each parent (e/e), so he does not have as much white fur as a stereotypical-looking double merle like his brother Martin, but his merle is not completely hidden since he has such long allele lengths.

Each puppy in Aston's litter had a 25% chance of inheriting merle from both parents like Aston. Aston's brother Martin was also born fully blind and fully deaf, while another brother was born deaf and blind in one eye. Two littermates were unimpaired.

The genetics of merle are very complex and there is misinformation everywhere. I have done my best to explain merle in a way that does not require an extensive background in genetics without watering it down too much. To learn more about merle, please visit Mary Langevin's website: Merle - SINE Insertion from Mc - Mh.