M-locus (merle alleles: Mh, M, Ma+, Ma, Mc+, Mc, m)*

The genetic test for merle detects the following alleles on the M-locus:
Mh (harlequin merle), M (classic merle), Ma+ and Ma (atypical merle), Mc+ and Mc (cryptic merle) and m (non-merle).

Harlequin merle (Mh):
The Mh-allele can create „harlequin merle“, as well as „minimal merle“ (only a small merled area on body, head or legs).

Classic merle (M):
The merle M-allele dilutes random parts of the coat colour to a lighter shade such as black to gray or brown to beige patches. The merled patches have irregular edges and are found anywhere (head, body and legs), resulting in the familiar merle coat pattern. Merle pattern can also affect the colour of the eyes, the nose and the paw pads. The merle M-allele is dominant to the wild-type non-merle m-allele. Therefore, a dog with one copy of the M-allele will express the merle pattern.
Homozygous merles (e.g. genotype Mh/Mh, Mh/M, M/M, formerly described as „double merles“) associate with health problems such as deafness, blindness and also sun sensitivity and skin cancer (due to the lack of UV-protecting pigmentation). Understandably, breeders should categorically avoid breeding Mh or M carriers to dogs that possess at least one Mh-, M-, Ma+- or Ma-allele.

Atypical merle (Ma+ and Ma):
Normally, one atypical merle Ma-allele won´t create a merle pattern on its own, but may create a „tweed“ or „patchwork“ merle pattern when combined with an M- or Mh-allele. Dogs with a homozygous Ma genotype exhibit a wide variety of phenotypes including diluted and blotched colours.

Cryptic merle (Mc+ and Mc):
Cryptic merle (Mc) is a short version of the merle insertion with no direct effect on the coat colour by itself. A lighter shades of the diluted areas as well as larger white areas are recorded for some dogs with the genotype M/Mc or M/Mc+. According to the actual state of knowledge cryptic merle is not associated with health problems.

Hidden merle and/or masked merle:
Merle can be hidden by certain phenotypes (e.g. recessive red/yellow/cream or extreme white spotting). That means the genotype M/m or Mh/m is not visible and a high-risk to breed homozygous merle (M/M) puppies by mistake exists.
Therefore, genetic testing for merle is strongly recommended in any breed or strain with merle before mating.

Mosaic merle:
Merle mosaics consist of more than three alleles (e.g. m plus at least two different merle alleles). True mosaicism might be difficult to detect, especially when additional merle alleles are present in certain sample materials only (e.g. in semen or egg cells). Mosaicism might create differing merle phenotypes like “minimal merle” or „tweed“.

Summary:
The test detects: Mh (harlequin merle), M (merle), Ma+ and Ma (atypical merle), Mc+ and Mc (cryptic merle) and m (non-merle) as well as mosaics of these alleles. Possible mating partners for merle-coloured dogs should be tested free for Mh, M, Ma+ and Ma at least, to avoid breeding of homozygous merle puppies and correlated health issues.