M-Locus Merle (Merle / Cryptic Merle)
Merle is a coat pattern found in Australian Shepherds and a number of other dog breeds. The phenotype is inherited as an autosomal, incompletely dominant trait. The Merle gene creates mottled patches of color in a solid or piebald coat, blue or odd-colored eyes, and can affect skin pigment as well. Animals that are “double Merle” a common term used for dogs that are homozygous for Merle (MM) are predominantly white and prone to several health issues. Chances of having puppies with health issues is more typical when two Merles are bred together, so it is recommended that a Merle dog only be bred to a non Merle, non cryptic Merle dog. Many solid dogs are actually cryptic or phantom merles and can produce both Merle and double merles.
Merle can affect all coat colors. Recessive red dogs can also 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 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, both dark-eyed, blue-eyed, and odd-colored eyes are possible. Color on paw pads and nose may be mottled pink and black.
A mobile genetic unit called a Retrotransposon insertion in premelanosome protein (PMEL17) or silver locus protein homolog (SILV) is responsible for Merle patterning in a number of domestic dog breeds. Merle is actually a heterozygote of an incompletely dominant gene. If two (Mm) dogs are mated, there on average 25% of the puppies will be (MM) homozygous or “double merles”, which is the common term for dogs that are homozygous for Merle. A high percentage of double Merle puppies have vision or hearing deficiencies.
A cryptic or phantom Merle is a dog which phenotypically appears to be a non-merle or very faint patches of Merle that can go unnoticed. Animals that do not present the Merle phenotype may possess the Merle genotype and subsequently produce Merle offspring. These animals are known as cryptic Merles.
Animal Genetics accepts buccal swab, blood, and dewclaw samples for testing. Sample collection kits are available and can be ordered at Canine Test Now.
Testing Is Relevant for the Following Breeds:
All Dog Breeds , American Cocker Spaniel , Australian Shepherd , Border collie , Cardigan Welsh Corgi, Chihuahua , Dachshund , Old English Sheepdog (Bobtail), English Bulldog, French Bulldog, Great Dane, Shetland Sheepdog (Sheltie), Rough Collie, Smooth Collie, Pomeranian, Pyrenean Shepherd, Bobtail, American Staffordshire Terrier, Hungarian Mudi, Catahoula Leopard Dog, Norwegian Dunker, Beauceron, Bergamasco.
Animal Genetics offers DNA testing for (PMEL17) mutation. The genetic test verifies the presence of the mutation and presents results as one of the following:
|M/M||Double Merle||Dog carries two copies of the dominant “M” allele. The dog is considered an affected, “double Merle” because M/M dogs can be affected by deafness and ocular defects. M/M dogs will always pass on a copy of Merle to their offspring.|
|M/m||Merle||Dog has one copy of the “M” Merle allele and one negative “m” copy of Merle allele. The dog can pass either allele on to any offspring.|
|m/m||Negative||Dog has two copies of the recessive “m” allele and is negative for Merle. The dog will always pass on a negative copy of the Merle allele to all offspring.|
|CR/CR||Double Cryptic||Dog carries two copies of the Cryptic “CR” allele.|
|m/CR||Merle||Dog has one copy of the Cryptic “CR” allele and one negative “m” copy of Merle allele. The dog can pass either allele on to any offspring.|
|m/m||Negative||Dog is negative for Cryptic “C” allele. The dog will always pass on a negative copy of the Merle allele to all offspring.|
|M/CR||Merle/Cryptic||Dog carries one copy of the dominant “M” allele and one Cryptic Merle allele. The dog is considered a Merle, and caries Cryptic Merle. The dog can pass either allele on to any offspring|
BMC Vet Res. 2006 Feb. 27;2:9. Coat colour in dogs: identification of the Merle locus in the Australian shepherd breed. Hédan B1, Corre S, Hitte C, Dréano S, Vilboux T, Derrien T, Denis B, Galibert F, Galibert MD, André C.