Merle is a coat pattern in dogs that creates a mottled patchwork of diluted and full-colored fur, often accompanied by blue or multi-colored eyes. It’s not a color itself but a genetic modifier that lightens patches of a dog’s base coat, producing that distinctive dappled or marbled look seen in Australian Shepherds, Great Danes, and about a dozen other breeds. The pattern is striking, but it comes with important health implications, especially when two merle dogs are bred together.
How the Merle Pattern Looks
Merle dilutes only the black-based pigment in a dog’s coat, called eumelanin. The result is irregular patches of full color scattered over a lighter, diluted background. A “blue merle” dog, for example, has a gray-blue base with dark black or slate patches. A “red merle” starts with a lighter reddish base and carries darker red or brown patches. The pattern is never uniform. Each merle dog has a unique arrangement of spots and swirls, almost like a fingerprint.
The dilution effect extends beyond the coat. Merle commonly affects the skin, producing mottled or “butterfly” noses with mixed pink and dark pigment. It also affects the eyes: many merle dogs have striking blue irises, or one blue eye and one brown eye (heterochromia). Some have a single eye that’s partly blue and partly brown. These eye color variations happen because the same pigment dilution that lightens the coat also reduces pigment in the iris.
The Genetics Behind the Pattern
Merle traces to a single mutation in a gene called PMEL, which builds a protein essential for depositing dark pigment inside cells. Specifically, a small piece of repetitive DNA (called a SINE element) has inserted itself into the PMEL gene. This inserted DNA has a repeating tail of genetic “letters,” and the length of that tail is what controls the intensity of the merle effect.
When the tail is short, the gene still works normally and the dog looks solid-colored. As the tail gets longer, it increasingly disrupts the gene’s instructions, producing an abnormal version of the PMEL protein that can’t build proper pigment structures inside the cell. The longer the tail, the more pigment production is disrupted, and the lighter the coat becomes. At the extreme end, very long tails can produce dogs that are almost entirely white.
This is why merle exists on a spectrum rather than as a simple on-or-off trait. Dogs at the lower end of the range may carry the merle mutation but look nearly solid-colored. These are called “cryptic merles,” and they pose a unique challenge for breeders because they don’t look like merles but can still pass the gene to their puppies.
Cryptic Merle: The Hidden Carrier
Cryptic merles carry a shorter version of the SINE insertion, meaning their PMEL gene still functions well enough to produce a normal or nearly normal coat. These dogs may show no visible merle patterning at all, or just a tiny patch of diluted fur that’s easy to miss. Without DNA testing, a cryptic merle can be mistaken for a solid-colored dog.
This matters because if a cryptic merle is unknowingly bred to a visible merle, the pairing can produce “double merle” puppies, which inherit two copies of the mutation. Because the repeating tail can change length when passed from parent to offspring, a cryptic merle parent can produce puppies with a much more intense version of the gene than they carry themselves. DNA testing at the merle locus is the only reliable way to identify these hidden carriers before breeding.
Health Risks of Double Merle
A dog that inherits two copies of the merle gene (one from each parent) is called a double merle, sometimes written as “MM.” These dogs have dramatically reduced pigment throughout their bodies, often appearing mostly or entirely white. The lack of pigment isn’t just cosmetic. Pigment-producing cells play critical roles in the development of the inner ear and the eye, and when those cells are absent or dysfunctional, the consequences can be severe.
A study published in the Journal of Veterinary Internal Medicine found that 54.6% of double merle dogs had some degree of deafness, either in one ear or both. Among double merles specifically, 10% were deaf in one ear and 15% were completely deaf. Single merles (carrying just one copy) had a much lower overall deafness rate of about 4.6% unilateral and 4.6% bilateral. The difference was statistically significant.
Vision problems are also common in double merles. The merle gene is associated with a range of eye abnormalities, including abnormally small eyes (microphthalmia) and structural defects in the eye’s interior. Research on Australian Shepherds found that microphthalmia and related eye anomalies occurred more frequently in merle dogs with predominantly white coats, reinforcing the connection between extreme pigment loss and developmental problems. Some double merles are born both deaf and blind.
Breeds Where Merle Is Recognized
The merle pattern is accepted in the breed standard for a number of dogs registered with the American Kennel Club:
- Australian Shepherd
- Miniature American Shepherd
- Border Collie
- Collie
- Shetland Sheepdog
- Cardigan Welsh Corgi
- Great Dane
- Dachshund
- Catahoula Leopard Dog
- Chihuahua
- Pomeranian
- Pyrenean Shepherd
- Mudi
Merle has also appeared in breeds where it isn’t traditionally recognized, such as French Bulldogs, Poodles, and Pit Bulls. In these breeds, the presence of merle typically indicates crossbreeding at some point in the dog’s lineage, and many breed clubs and registries do not accept merle-patterned dogs in these breeds.
Breeding Practices and Policies
Because of the serious health risks to double merle puppies, breeding two merle dogs together is widely discouraged. The United Kingdom Kennel Club and the Australian Shepherd Club of the UK have outright banned merle-to-merle breeding, refusing to register any offspring from such pairings. In the United States, the Australian Shepherd Club of America recommends against the practice but has not implemented a formal ban.
Responsible breeders pair a merle dog with a non-merle partner, which ensures no puppy can inherit two copies of the gene. This approach produces litters where roughly half the puppies will be merle and half will be solid, with none at risk for double merle complications. DNA testing before breeding is especially important given the existence of cryptic merles, which can carry and pass on the gene despite appearing solid-colored. A simple genetic test can identify a dog’s merle status and the approximate length of its SINE insertion, giving breeders the information they need to make safe pairing decisions.
One important quirk of the merle mutation: it only affects eumelanin, the black-based pigment. Dogs that are genetically unable to produce eumelanin (those with two copies of the “e” variant at the MC1R gene, which produces all-red or all-cream coats) won’t display the merle pattern even if they carry the gene. These dogs can still pass merle to their offspring, creating another scenario where genetic testing is essential to avoid accidental double merle litters.