Merle coat pattern is inherited as an autosomal, incompletely dominant trait. Merle is a coat pattern characterized by irregularly shaped patches of diluted pigment and original colour and is seen in many dog breeds. Merle gene not only alters base coat colour, but also modifies eye colouring, colouring on the nose, paw pads and skin. The Merle gene modifies the dark pigment in the eyes, thus merle dogs typically have blue or partially blue eyes. Because of random modifications, dark-eyed, blue eyed and odd-coloured eyes are possible. Colour on paw pads and nose may be mottled pink and black.
Merle dogs can have one copy of normal allele m and one copy of merle allele M (merles - heterozygous dogs carrying alleles M/m) or two copies of merle alleles (double merles- homozygous dogs carrying alleles M/M). Heterozygous merle dogs typically have mottled patches of colour in a solid or piebald coat, and can have blue or partially blue eyes. They usually have no hearing or eye problems. Double merle dogs are predominantly white and often have a wide range of auditory and ophthalmologic defects and multiple abnormalities of skeletal, cardiac, and reproductive systems. Breeding between two merles is thus discouraged to avoid producing double merle offspring.
Merle affects only black pigment eumelanin, thus any black, liver, blue or isabella coat colour, and also eyes and nose will be merled. Red pigment phaelomelanin is not affected by merle at all and will appear as normal, thus red (genotype ee) dogs that are genetically merles will appear as normal and will be hidden merles, only the eyes can be blue. They do not express merle but can produce merle offspring. Similar case of hidden merle is with sable dogs (having one allele Ay) where only the tip of the hair can be merled while producing eumelanin and it is difficult to visually distinguish a merle from non-merle dog.
Another type of hidden merles are cryptic merles. Cryptic merle dogs phenotypically appear to be a non-merle or have slight merle coloration that can go unnoticed. These dogs can be heterozygous or homozygous for shorter version of merle gene- allele Mc (Mc/m, Mc/Mc) and have no health problems. Because this dogs are normal coloured they can be incorrectly registered as non-merles.
Inheritance of merle is genetically unstable for both M and Mc alleles. In the offspring allele M can occasionally undergo reduction to produce Mc, while Mc may expend and revert to M. When breeding cryptic merle (Mc/m) with a non-merle (m/m) the offspring can have following genotypes: cryptic merle/non merle (Mc/m), merle/non merle (M/m), non merle/non merle (m/m).
Because of autosomal dominant mode of inheritance double merle, merle, double cryptic merle and cryptic merle coat pattern is expressed in animals that carry a mutation (one or both affected alleles). Because of potential health risks DNA testing for merle gene is recommended to reveal the genetic background of dogs for the merle gene variants for those breeds where this colour dilution pattern is present. With the aim of prevention of possible animal suffering it is advised to avoid breeding of double merles, merles and cryptic merles with merles, and preferred to breed with non-merle (m/m) dogs.
Inheritance: incomplete autosomal dominant- read more
Mutation: SILV gene
Genetic test: Affected or carrier can be identified by genetic testing at any age. The PCR method used for testing is extremely accurate and allows complete differentiation between affected animals, carriers and healthy dogs.
Affected breeds: Australian Shepherd, Border Collie, Shetland Sheepdog, Collie, Catahoula Leopard Dog, Cardigan Welsh Corgi, Dachshund, Great Dane, English Bulldog, French Bulldog, Chihuahua, American Pit Bull Terier, American Staffordshire Terrier, Beauceron, Poodle, Pyrenean Shepherd, Old English Sheepdog, American Cocker Spaniel, Pomeranian, Hungarian Mudi, Norwegian Dunker Hund
Sample: EDTA whole blood (1.0 ml) or buccal swabs. Detailed information about sampling can be found here.