Merle is a pattern in a dog's coat, though is commonly incorrectly referred to as a color.[1]
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Merle can affect all coat colors but the colors most commonly seen that are affected are brown and black, when affected by merle they are usually called liver and blue, though some call liver red. 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, it is not visible at all.[2][3] Combinations such as brindle merle exist, but are not typically accepted in breed standards.
In addition to altering base coat color, merle also modifies eye color and coloring on the nose and paw pads. The merle gene modifies the dark pigment in the eyes, occasionally changing dark eyes to blue, or part of the eye to be colored blue. Since merle causes random modifications, however, both dark-eyed, blue-eyed, and odd-colored eyes are possible.[4] Color on paw pads and nose may be mottled pink and black.[5]
Merle is a distinguishing marking of several breeds, particularly the Australian Shepherd, and appears in others, including the Koolie, German Coolies in Australia, the Shetland Sheepdog, various Collies, the Welsh Corgi (Cardigan), the Pyrenean Shepherd, the Bergamasco Sheepdog, the Old English Sheepdog, and Catahoula Leopard Dog. In Dachshunds the merle marking is known as "dapple".[6] It is also present in the Pomeranian and Chihuahua, but is a disqualification according to the FCI standards. In the American Pit Bull Terrier, and Cocker Spaniel breeds it is not a recognized color. The merle gene also plays a part in producing harlequin Great Danes.
Merle is actually a heterozygote of an incompletely dominant gene.[7] If two such dogs are mated, on the average one quarter of the puppies will be "double merles", which is the common term for dogs homozygous for merle,[8] and a high percentage of these double merle puppies could have eye defects and/or be deaf. Knowledgeable breeders who want to produce merle puppies mate a merle with a non-merle dog; roughly half the puppies will be merles without the risk of vision or hearing defects associated with double merle dogs.[9]
In January 2006 scientists at Texas A&M University announced the discovery of a mobile genetic unit called a retrotransposon, responsible for the merle mutation in dogs.[10]
A phantom merle or cryptic merle is one with such small patches of merle—or none at all—that it appears to be a non-merle.[11] 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.[12] In America, a dog with the phantom merle coloring is described as being "cryptic for merle."
The merle gene is often associated with congenital deafness, with merle dogs being more likely than other dogs to be born deaf. Dogs with two copies of the merle gene (homozygous merle) have an even higher chance of being born deaf.[13] The suppression of pigment cells (melanocytes) in the iris and in the stria vascularis of the cochlea (inner ear) leads to blue eyes and deafness. An auditory-pigmentation disorder in humans, Waardenberg syndrome, reflects some of the problems associated with heterozygous and homozygous merle dogs and genetic research in dogs has been undertaken with the goal of better understanding the genetic basis of this human condition.[10]
Dogs who are homozygous for the merle pattern gene often have visual and auditory deficits.[11] These dogs are sometimes referred to as 'double merle' and sometimes incorrectly referred to as 'lethal white.' Ocular defects include micropthalmia, conditions causing increased ocular pressure, and colobomas, among others.[14] Double merle dogs may be deaf or blind or both, and can carry ocular defects in blue or colored eyes.[15] Currently no studies have been done to prove whether or not the merle gene affects the eyes, causing blindness.
One study done by a German researcher showed that out of 38 dachshunds, they found hearing loss in 54.6% of double merles, and 36.8% of single merles. However, this percentage was based on one of the dogs and its hearing range, not the standard. The actual results were 9.1% of the double merles was deaf, that percentage being one, and 0% of the single merles were deaf.[16] Another study done by Texas A&M University found that out of 22 double merles, 8 were completely deaf and two were deaf in one ear. Out of 48 single merles, only one was deaf in one ear, none were completely deaf.[17] Another study done with 70 dogs, 15 of them were Catahoula Curs and 4 of them were deaf, whereas the other 86% of double merles in other breeds were deaf.[17]
Deaf, blind, and deaf and blind dogs can have good lives when properly cared for. There are a variety of internet groups dedicated to supporting carers of such dogs. Deaf dogs can compete successfully in agility[18] and there are many anecdotal reports of deaf/blind dogs earning their Canine Good Citizen certification, working as therapy dogs, and competing in dog sports like tracking or Nosework.