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A practical guide to the M locus (PMEL), cryptic merle, mosaicism and DNA testing for dog breeders.
Merle creates a marbled coat pattern with patches of lighter and darker pigment. But appearance alone does not tell breeders everything about a dog’s merle genetics. Some dogs carry merle without obvious markings, and not every merle variant acts the same way.
The most important rule: use the actual PMEL DNA test results of both parents, not only their coat colors when planning a mating.
Merle is associated with an extra piece of DNA inserted into the PMEL gene, also called the M locus. PMEL helps form dark pigment. When the insertion affects the gene, parts of the coat can become lighter.
The inserted sequence is not always the same length. Shorter versions often have little visible effect; longer versions are generally associated with more noticeable merle patterns. But there are exceptions, so the length does not guarantee a particular appearance.
A detailed merle DNA test can report which length group a dog carries. These names make it easier to understand a result, but the borders between groups are guidelines rather than absolute rules.
|
Result |
Common name |
What you may see |
|
m |
Non-merle |
No PMEL merle insertion |
|
Mc / Mc+ |
Cryptic merle |
Often looks non-merle |
|
Ma / Ma+ |
Atypical merle |
Unusual dilution, faint or irregular pattern |
|
M |
Classic merle |
Typical marbled patches |
|
Mh |
Harlequin merle |
Often more complex merle-associated patterns |
Important: these allele labels do not replace the measured base-pair result. Different published classification systems also place some boundaries slightly differently.
Yes. Three terms are especially useful to distinguish:
|
Term |
Plain-language explanation |
Why breeders care |
|
Cryptic merle |
The dog carries a relatively short insertion that usually creates little or no visible pattern. |
It can look identical to an m/m dog but has a different DNA result. |
|
Hidden merle |
The dog carries a merle allele, but its other coat-color genetics hide the pattern (for example, some e/e red or yellow dogs). |
A visually non-merle dog may pass on a classic merle allele. |
|
Minimal merle |
Only one tiny area of the coat shows obvious merle. |
A small patch does not prove that the insertion is short. |
Example: a red or cream dog may carry m/M without showing obvious coat patches because merle mainly affects eumelanin (dark pigment), not pheomelanin (red/yellow pigment).
Mosaicism means that different groups of cells in one dog can contain different lengths of the merle insertion. A detailed test may therefore detect more than two merle-related signals. This does not mean every cell has three copies of the gene.
Some mosaic variants can potentially be passed to puppies if they are present in reproductive cells. A reported mosaic percentage from a cheek swab is NOT the same as the chance of passing that allele to offspring.
Ballif et al. (2018) detected more than two PMEL allelic signals in 24 of 175 dogs studied; their sample was selected and should not be treated as a prevalence estimate for all dogs.
A double-merle dog inherits a merle-associated insertion from each parent. The classic example is breeding two m/M dogs. On average, each puppy then has a 25% chance of inheriting M/M.
Dogs with two strongly expressed merle alleles have increased risk of congenital hearing and eye abnormalities, including deafness and abnormal eye development. This is why classic merle × classic merle matings should be avoided.
Not every pair of PMEL insertions behaves identically: two very short cryptic alleles can differ substantially from two classic alleles. Nevertheless, avoid calling a mating “safe” merely because one result is labeled cryptic or atypical.
|
Parents |
Expected result / decision point |
|
m/m × m/M |
About 50% m/m and 50% m/M; no puppy inherits two merle insertions from these parental genotypes. |
|
m/M × m/M |
25% m/m, 50% m/M, 25% M/M; avoid because of the double-merle risk. |
|
m/Mc × m/M |
A quarter may inherit Mc/M; exact insertion sizes and assay limitations matter. |
|
m/Ma × m/M |
Some puppies may inherit two merle insertions; specialist interpretation is recommended. |
|
Mosaic result × merle |
Transmission may be harder to predict; seek detailed laboratory interpretation. |
These are expected probabilities per puppy, not guaranteed litter ratios or estimates of clinical disease risk. They assume stable alleles and no relevant germline mosaicism.
Occasionally, yes. The repetitive part of the PMEL insertion can become shorter or longer. Ballif et al. documented a father with M268 and a puppy with M258, consistent with contraction. However, such changes are not inevitable, and there is no reliable prediction that a particular cryptic allele will become classic merle.
Ideally, the laboratory should identify the merle insertion length, classify the allele using its stated system, and explain whether additional signals suggest mosaicism. Not all tests have equal sensitivity to low-level mosaicism, and a cheek swab cannot fully describe every reproductive cell.
Yes, if one carries a hidden, cryptic, or relevant mosaic merle variant. Two dogs genuinely confirmed m/m would not ordinarily be expected to produce a PMEL-merle puppy.
No. A cryptic merle dog has a PMEL insertion; an m/m dog does not. Their coats may look the same.
No. Visible patch size cannot reliably identify the insertion length or rule out health concerns for offspring.
No. Increased genetic risk does not mean every affected-genotype dog has clinical abnormalities. The risks are serious enough to avoid conventional merle × merle matings.
No. Blue iris pigmentation alone does not diagnose poor vision. Hearing can be evaluated using BAER testing; eyes require veterinary examination when indicated.
Veterinary genetics disclaimer: DNA tests identify detectable variants in the sampled tissue; they cannot guarantee appearance, clinical status, or complete representation of germline mosaicism. Test interpretation does not replace appropriate veterinary hearing and eye examinations.