The "blue" merle is genetically a black dog.
Technically a coat pattern, the "merle" gene has a significant impact on the color of a dog: black coats become "blue" merles, blue coats become blue or lilac merles, chocolate coats become "red" merles, etc. In addition to the bleaching effect that the pattern creates, "double" merles, which have two merle parents, appear to be white dogs with colored patches. The merle is a complex issue in coat genetics, with some associated complications, as well.
The Genetics of the Merle Coat
The merle gene causes an irregular dilution upon the dog's visible coat color. The irregularity of this dilution produces what appears to be a series of dark color patches on a lighter colored field. According to Shetland sheepdog breeder Sue Ann Bowling, "the lightening seems to work primarily on the black pigment in the coat, so any tan on the face stays even." Because genes for the liver/chocolate, blue dilute and even the sable coat colors all fall on the same position on the chromosome strand, called a "locus," they are all affected by the merle modifier. The dog with a merle coat may also have blue eyes or eyes that are both blue and brown ("split" or "marbled"), due to the gene's effects on the eye pigment.
The "Blue" Merle and other Merles
In reality, the familiar "blue" merle is a dog with a black coat that has been modified by the merle gene. This misnomer has led to some creative names such as "slate merle" for dogs with the blue-gray base coat color. These dogs have dark blue markings on a light blue field.
The general public is less familiar with merles of colors other than blue. These colors include "red" merles (chocolate/liver markings on a brown field), "lilac" merles (pale red on a light brown field), and sable merle. Interestingly, long-coated sable merles are likely to be "cryptic" merles, undetectable by casual observation, because sable is already a shaded color. Some mottling on the backs and tips of the dog's ears may reveal it as a sable merle to the knowledgeable observer.
The Double Merle
The merle gene is dominant to other coat colors and patterns. Due to its nature as a diluting factor, when one merle is bred to another, it creates a white dog with small patches of color. These dogs, called "double merles," are typically unhealthy. Sue Ann Bowling states that double merles are "usually deaf or blind and often with other physical problems. Some [double merle] puppies are born completely without eyes." Ethical breeders will not breed double merles for sale to the general public.
Breeds with Merle Coats
Many of the herding breeds have merle coats permitted by their standards: collies (rough and smooth), border collies, Pembroke and Cardigan Welsh Corgis, Louisiana Catahoula leopard dogs, and Australian shepherds are among them. However, the merle coat has also appeared in unexpected breeds, such as Dachshunds (where it is known as "dapple"), Chihuahuas and Papillons.
Similar Coat Colors and Patterns
Although the merle gene has a unique expression, some other mottled patterns produce similar effects. The AKC Glossary also describes the Harlequin ("patched or pied coloration," usually describes Great Danes), mottled ("Pattern of dark roundish blotches on a lighter background"), and roan ("fine mixing of colored hairs with white hairs"). Some dogs with roan markings also have other patches of color, as seen in German shorthaired pointers and Australian cattle dogs. Such dogs can sometimes be mistaken for merles.
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