Coat colour merle testing as a part of our services

Did you know that the MYDOGDNA™ test also features screening for merle? Please visit your dog’s result profile to see if you already have a merle result available.

Merle coat colour is a combination of patches of diluted base colour and areas of full pigmentation. The merle allele dilutes dark eumelanin pigment to a lighter shade, for example black to grey or brown to beige, while leaving patches of the original dark colour remaining. The merle pattern is often irregularly spread over the dog’s body. Red/yellow phaeomelanin pigmentation is not affected by merle. The eyes of a merle dog can be entirely or partly blue. Similarly, the nose pigment may be fully or partially lightened to pink shade.

The merle trait is inherited in an autosomal, incompletely dominant fashion, meaning that carrying only one copy of (= heterozygosity for) the genetic variant responsible for merle patterning is sufficient for a dog to show merle coat colour (other coat color modifiers permitting). Dogs that carry two copies of (= are homozygous for) the merle (M/M) variant are usually predominantly white. These so called double merle dogs (M/M) are prone to several health concerns, such as deafness, eye development defects and blindness, and may be stillborn. In certain breeds, also heterozygous merle (M/m) dogs may suffer from similar health issues. Mating of two merle is not recommended to avoid the health concerns associated with double merle dogs.

Shifting focus to molecular genetics, the variant responsible for merle coat colour is a so called insertion mutation in the SILV gene. It is now known that there are varying lengths to the insertion, enabling subclassification of different merle alleles. In general, a “cryptic merle allele” (Mc) is a merle insertion shortened to the extent that it is no longer able to cause a coat pattern change even when it is present in the genome of a dog. Therefore, dogs carrying “cryptic merle” show only very slight merle colouration or no merle colouration at all. The cryptic merle allele is currently not known to be associated with health concerns. The merle test included in the MYDOGDNA™ testing service is able to state whether a SILV gene merle-associated insertion is present in the genome of the tested dog, but does currently not report separately on different lengths of the insertion.

Finally, the merle coloration is not always visible due to other factors modifying coat colour. For example, the recessive red (e/e genotype) and sable (ay/- genotype) coat color may hide the merle phenotype, or the merle coloration is seen very subtly for example in the eyes. Such dogs are often called “phantom” or “ghost” merles. Genetic testing is the most accurate means of confirming that a dog carries merle.