Merle M locus (PMEL-related) - Dog

The Merle M-locus test in dogs analyses variation in the length of a SINE insertion at the boundary of intron 10 and exon 11 of the PMEL gene. This length variation influences how strongly the merle pattern is expressed. The test identifies the main merle alleles: m (non-merle), Mc and Mc+ (cryptic merle), Ma and Ma+ (atypical merle), M (classic merle), and Mh (harlequin merle).

Merle causes irregular dilution of dark pigment, which can create a marbled or patchy appearance in the coat, skin, and sometimes the eyes. However, expression varies widely between dogs. Some dogs show an obvious merle pattern, while others genetically carry a merle variant but show little or no visible sign of it. This is especially relevant for cryptic merle and some atypical merle variants.

That is why DNA testing is so valuable. A dog that appears visually non-merle may still carry a merle allele and pass it on to offspring. Without genetic testing, this can lead to unexpected merle puppies or breedings with an increased genetic risk. This test helps distinguish visible, subtle, and hidden forms of merle and supports more informed breeding decisions.

This trait is inherited in an autosomal incompletely dominant manner. Dogs with one merle-associated allele may show a merle pattern, although the degree of expression can vary. Dogs with two merle-associated alleles, often referred to as double merles, have a clearly increased risk of extensive depigmentation and a predominantly white coat.

This is also why certain combinations are considered high risk. When two merle carriers are bred together, some puppies may inherit two merle alleles. These dogs have an increased risk of hearing problems and eye defects. In practice, double merles are more often associated with deafness, microphthalmia, developmental eye abnormalities, and impaired vision. The risk is influenced not only by genotype, but also by the degree of white patterning and by other genetic factors affecting pigment distribution.

The result of this test is therefore relevant not only for coat color prediction, but also for responsible breeding management. It helps determine whether a dog carries a hidden merle variant, whether a planned mating may create an undesirable risk, and whether an unusual coat pattern has a genetic explanation. This makes the test especially useful for breeders, veterinarians, and owners who want to reduce surprises in a litter.

Other loci can influence how clearly merle is visible. For example, the E-locus and loci involved in white spotting can affect whether enough dark pigment is present for the merle pattern to be seen clearly. As a result, a dog may be genetically merle while the pattern remains subtle or largely masked.

It is also important to know that the merle region is not completely genetically stable. In rare cases, small length changes may occur, which helps explain why merle inheritance and expression can be more complex than coat appearance alone would suggest. DNA testing therefore provides much more reliable information than visual assessment alone.

• Identifies whether a dog is non-merle or carries a cryptic, atypical, classic, or harlequin merle variant
• Helps detect hidden merle in dogs without an obvious visible pattern
• Supports breeding decisions by helping identify higher-risk merle x merle combinations
• Clarifies coat color expectations and helps reduce unexpected merle offspring
• Provides extra genetic insight for subtle, atypical, or difficult-to-interpret coat patterns