M locus (merle incl. base length)

Coat pattern:
The typical merle colouring is characterised by a random, patchy lightening of colour, which gives the coat a speckled appearance. Only the areas of the coat that are coloured by eumelanin (black/brown pigment) are affected. Merle has a dominant effect on these areas. Coat areas coloured with the red pigment phaeomelanin are not affected by merle. A black dog with merle markings is called a blue merle, a brown dog (in the B locus b/b = brown + brown nose) with merle markings is called a red merle.

Health risks:
Double merle dogs that have two genetic predispositions for the merle factor (M/M) can have severe health problems, such as deafness, blindness, sun intolerance, skin cancer, skeletal deformities, etc.). When breeding with a Merle carrier, it is essential to ensure that a non-Merle (m/m) dog is chosen for the mating. If two merle dogs are mated, there is a high risk of producing double merle puppies.

Genetic background:
The merle factor is characterised by a genetic insertion, i.e. an additional gene sequence has been inserted within the affected gene PMEL17. The additional sequence can vary in length from dog to dog. It is assumed that the longer this insertion is, the stronger the merle pattern will be characterised by a more intense lightening.

If the dog has a very short insertion variant, it is referred to as cryptic merle or phantom merle (Mc). A dog usually carries this gene variant in a hidden form, with no obvious merle markings or only very small, inconspicuous areas on the body indicating merle. Even dogs that are double cryptic merle (Mc/Mc) can look inconspicuously like non-merle dogs. Only a genetic test can clarify whether a dog is a Mc carrier.

Another special feature of the Merle insertion is the instability of the additional sequence length. This means that a short Mc variant can become a long Merle variant (M) and vice versa. This must be taken into account especially when mating Merle dogs (M/m) with optical non-Merle dogs (Mc/m or Mc/Mc). In this combination, double merle offspring may be produced if a Mc variant turns into a merle variant (M) in the next generation. Even the M/Mc genotype (merle/cryptic merle) can already result in health restrictions. Therefore Mc carriers should only be bred to non merle dogs (m/m).

When breeding with merle dogs, it is advisable not to rely on the visual coat pattern. A genetic M-Lokus (Merle) test provides clarification and supports healthy breeding.