Merle is a colour combination in dogsâ€™ coats. It is a solid base color (usually red/brown or black) with lighter blue/gray or reddish patches, which gives a mottled or uneven speckled effect.
Although most breeds that can have merle coats also typically have white markings (such as around the neck, under the belly, and so on), and often tan points (typically between the white and the darker parts of the coat), these are separate colors from the merle; some dogs do appear completely merled with no white or tan markings.
Merle is a distinguishing marking of several breeds, particularly the Australian Shepherd, and appears in others such as German Coolies in Australia, the Shetland Sheepdog, various Collies, the Welsh Corgi (Cardigan), the Pyrenean Shepherd, the Catahoula Leopard Dog, the Koolie, the Old English Sheepdog and others.
Merle is actually a heterozygote of an incomplete dominance gene. If two such dogs are mated, on the average one quarter of the puppies will be double merles and some percentage of these double merle puppies could have eye defects and/or could be deaf. Knowledgeable breeders who want to produce merle puppies mate a merle with a nonmerle dog; roughly half the puppies will be merles without the risk of vision or hearing defects.
In January 2006, scientists at Texas A&M University announced the discovery of the mobile genetic unit, a retrotransposon, responsible for the merle mutation in dogs.
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