At one time, dogs with blue merle markings were considered mottled, brindle or spotted dogs. As canine geneticists have begun to unlock the mystery of this beautiful and unique coat, desired by many dog breeders and owners, they have come to understand that blue merle is not a color, but rather a coloring pattern.
The Black and Blue Dog
A blue merle is not a blue dog but rather a black dog whose coloring has been genetically diluted -- much like someone spilled bleach on him in the wash. Parts of his black coat remain intact, while other parts are lightened to a bluish-grey color. Patches of solid color are normally irregular in shape; they can be located anywhere on the dog's body.
Painting by Genetic Numbers
There are nine points on a dog's chromosomes, known as loci, that combine together and are responsible for the dog's ultimate coat coloring. Alleles reside at each locus on the chromosome, and every dog inherits pairs of chromosomes, one from each parent. Alleles can be dominant or recessive -- and the manner in which they're combined determines how a dog looks. The alleles for the merle color pattern are located at the M locus on the chromosome. Merle is dominant (M), and non-merle coloring is recessive (m). When a dog inherits a heterozygous pair (Mm or mM), the dog will exhibit the merle color pattern. Dogs inheriting a homozygous pair of dominant genes (MM) are called double merle and usually are solid white in color. The merle gene affects only the eumelanin pigments, meaning any black, liver, blue or Isabella coloring can appear as merle coloring. Phaeomelanin pigment such as red is not affected by the merle gene and will appear solid.
Different Shades of Blue
Blue merles can exhibit a range of coat markings. Some blue merle dogs will have a mostly gray-blue coat with a few small, black patches. The most common merle pattern and the one described in most breed standards is a coat that is approximately 50 percent merle, or gray-blue, and 50 percent black patches. Blanketed blue merles have large black patches and smaller areas of merle coloring. Finally, dogs appearing mostly or entirely black with little or no visible merle coloring are known as cryptic merles.
Not every breed will exhibit merle coloring. The gene is present in the Australian shepherd, border collie, old English sheepdog, mudi, cardigan Welsh corgi, rough and smooth coated collie, Shetland sheepdog, Pyrenean shepherd, dunker and Catahoula leopard dog. Recently, the merle gene has been bred into the Chihuahua, American Staffordshire terrier, American cocker spaniel and the Pomeranian. The merle gene is also responsible for the dappling you see in the dachshund and in the harlequin pattern of the Great Dane.