Merle (dog coat)

Merle is a genetic pattern in a dog's coat and alleles of the PMEL gene. It results in different colors and patterns and can affect any coats. The allele creates mottled patches of color in a solid or piebald coat, blue or odd-colored eyes, and can affect skin pigment as well. Two types of colored patches generally appear in a merle coat: brown/liver (red merle) and black (blue merle). Associated breeds include Carea Leonés, Australian Shepherds and Catahoula Leopard Dogs. Health issues are more typical and more severe when two merle-patterned dogs are bred together.

Description Merle can affect all coat colors. The merle forms of brown are usually called "red" (though this is not correct; red and brown are genetically different), and black is called "blue" as lighter patches of black are formed throughout the coat and look slightly blue in color. Dogs who are recessive red can still be affected by merle, but the patches are either hardly seen or, if the dog is a clear recessive red, are not visible at all. Combinations such as brindle and sable merle exist, but are not typically accepted in breed standards. In addition to altering the base coat color, merle also modifies eye color and coloring on the nose and paw pads. The merle allele modifies the dark pigment, resulting in eyes or parts of eyes turning blue. Since merle causes random modifications, dark-eyed, blue-eyed and odd-colored eyes are possible. Pigmentation on paw pads and nose may be mottled by pink.

Breeds Merle is a distinguishing marking of several breeds, particularly the Australian Shepherd, Australian Cattle Dog, and Catahoula Leopard Dog, and appears in others, including the Miniature American Shepherd, the Koolie in Australia, the Shetland Sheepdog, various collie breeds, the Cardigan Welsh Corgi, the Pyrenean Shepherd and the Bergamasco Shepherd. In Dachshunds, the merle pattern is known as "dapple". In Beaucerons the merle pattern is known as "harlequin". The merle allele also plays a part in producing harlequin Great Danes. Depending on the breed, registry or country, the pattern may be register-able with a registry but not allowed in conformation, and the pattern may not be recognized by the breed standard, disqualifying it from showing. There may also be additional requirements for the pattern such as the dog being allowed to have the pattern but must have completely dark eyes with no blue in them. Merle is sometimes introduced to other purebred dog breeds through crossbreeding such as French Bulldogs, poodles and even American Bullies, but these dogs are not purebred. In 2020 the UK Kennel Club (the oldest in the world) banned the registration of merle poodles as it is not and never has been a colour pattern recognised in pure poodles by the club.. The Pembroke Welsh Corgi Club of America states that the only accepted Pembroke colors are red, sable, fawn, and black and tan, with or without white markings, and that “brindle and merle in either red or blue are NOT naturally occurring or accepted colors for Pembroke Welsh Corgis.”

Genetic basis

The merle allele is actually an incompletely dominant allele, and all dogs with standard merle coats are heterozygous, having only one copy of the merle allele. In normal breeding of a merle (Mm) and a non-merle (mm) dog, approximately half of puppies are expected to be merle. Merle can be tested for and identified through genetic testing. In 2006, scientists announced the discovery of a mobile genetic unit called a retrotransposon responsible for the various merle mutations in dogs. In 2018, a published paper identified six general categories of merle alleles that affect (or do not affect) the final coat color depending on the length of the SINE insertion within them.

Double merles If two heterozygote merle dogs (Mm) are mated, on average one quarter of the puppies will be born with two merle alleles (MM). These homozygous dogs are often called "double merles", or more rarely "lethal whites". These dogs may be entirely white, or present with predominantly white coats with some small patches of other colors. A high percentage of double merle dogs have eye defects or are deaf. Because of these health issues, merle to merle mating is avoided by breeders.

Cryptic merles A cryptic merle is one with such small patches of merling, or none at all, that it appears to be a non-merle. This is commonly seen in dogs who are recessive red, clear recessive reds in particular, though patches can still be seen in certain red dogs. In America, a dog with the phantom merle coloring is sometimes described as being "cryptic for merle". A cryptic merle bred to a merle may produce merles, cryptic merles, double merles or non-merles, depending on whether the parents were heterozygous or homozygous for the merle alleles. Another mutation for cryptic merle has been identified by Dr. Helena Synková, and has been given the working name of "atypical merle". It is recommended that if a breeder is unsure if their dog is merle or not, that they be tested for the merle allele lengths.

Modifiers Certain modifying genes work in tandem (co-dominant expressive) with the merle alleles to create a completely different look to the pattern. Often mistaken for a "double merle", a harlequin merle (or just harlequin), is a Great Dane that carries both the merle pattern allele and the co-dominant modifying gene for harlequin, PSMB7 (also known as H). This causes most or all of the diluted "blue" color to be replaced with white, resulting in a dog that is mostly white with black patches. All dogs exhibiting the harlequin pattern also have the merle allele. This pattern is found only in Great Danes, and is registered as harlequin.

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