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

The coat colour Merle (M) denotes a coat pattern in which both areas with fully pigmented coat and areas with diluted colour pigment occur. The Merle factor causes a colour dilution that is not distributed across the entire coat, but leaves irregular, torn-looking undiluted colour patches.

To date, the variants Mh (Harlequin Merle), M (Merle), Ma+ and Ma (atypical Merle), Mc+ and Mc (cryptic Merle) have been described, for which an insertion of variable length is responsible, which can occur in different allele variants:

• Harlequin Merle (Mh): The Mh allele can produce “Harlequin Merle” as well as “minimal Merle” (only a small region with Merle markings on the body, head, or legs).


• Classic Merle (M): The Merle allele M randomly dilutes parts of the coat colour to lighter shades, such as black to grey or brown to beige patches. The Merle patches have irregular borders and can occur anywhere (head, body, and legs), resulting in the well-known Merle coat pattern. The Merle pattern can also affect the colour of the eyes, nose, and paw pads. The Merle M allele is dominant over the wild-type non-Merle m allele. A dog with one copy of the M allele therefore shows the Merle pattern. In dogs with homozygous Merle (e.g. genotype Mh/Mh, Mh/M, M/M, formerly referred to as “double Merle”), health issues can occur (see also below).


• Atypical Merle (Ma+ and Ma): Normally, an atypical Merle Ma allele alone does not produce a Merle pattern, but in combination with an M or Mh allele it can produce a “tweed” or “patchwork” Merle pattern. Dogs with a homozygous Ma genotype show a variety of phenotypes, including diluted and mottled coat colours.


• Cryptic Merle (Mc+ and Mc):Cryptic Merle (Mc) is a short form of the Merle insertion that in itself has no direct effect on coat colour. In some dogs with the genotype M/Mc or M/Mc+, lighter shades of the diluted areas as well as larger white areas have been reported. According to current knowledge, cryptic Merle alone is not associated with health problems.

Special features:

• Hidden and/or masked Merle: Merle can be concealed by certain phenotypes (e.g. recessive red/yellow/cream or extreme white spotting). This means that the genotype M/m or Mh/m is not visible and there is a high risk of accidentally breeding homozygous Merle puppies.


• Mosaic Merle: Merle mosaics consist of at least three alleles (e.g. m plus at least two different Merle alleles). True mosaicism can be difficult to detect, especially if additional Merle alleles are present only in certain sample materials (e.g. in sperm or egg cells). Mosaicism can lead to different Merle phenotypes such as “minimal Merle” or “tweed”. Possible Merle mosaic results are indicated in the findings in square brackets, which identifies this allele as a minority component of the result.

In the findings, where Merle alleles are present, in addition to the exact designation of the variant the respective insertion length in base pairs (bp) is also stated. Please note, however, that the indication of insertion length in bp may vary both from laboratory to laboratory due to the test method used and its specifications, and also within the analysis of two samples from the same animal due to the specific genetics of the M locus. Comparative analyses based on the exact insertion length are therefore not useful either for two samples from the same animal or between the investigating laboratories.

In dogs with the genotype M/M (double Merle) as well as all genetic combinations of M or Mh with the alleles Mh, M or Ma, severe inner ear malformations can occur. These can lead to hearing impairment or even deafness. In addition, eye malformations, increased sensitivity to sunlight, and skin cancer (due to the lack of UV-protective pigmentation) can occur. These animals often have a very high proportion of white or are completely white in colour. A mating that can produce such puppies is considered Qualzucht under Section 11b of the German Animal Welfare Act (and current case law), and such a mating is therefore prohibited.

The risk of effects on health is classified according to the present genotypes (based on current scientific knowledge) as follows:

Since the expression of Merle colouration can also be limited to only small areas (minimal Merle) or can be masked by another colour (hidden Merle), a visual identification of these animals is not always possible. A genetic test is therefore always advisable if Merle is present or suspected in a line used for breeding.

Further information can also be found in our article Current classification of breeding with the Merle trait