M Locus · PMEL (SILV)

M Locus: Merle

Creates the mottled merle pattern through a variable-length DNA insertion. The insertion length determines patterning intensity — and breeding two merle dogs together risks producing double merles with serious deafness and blindness.

Alleles

m: Non-merle: No SINE insertion (roughly under 200 bp). No merle pattern. The dog does not carry merle and cannot produce merle puppies unless bred to a merle.
Mc: Cryptic merle: ~200–230 bp insertion. Usually produces little or no visible merle patterning — the dog can look solid. However, it IS a merle allele and MUST be treated as such for breeding safety purposes.
Mc+: Cryptic+: ~231–246 bp. Minimal patterning, can express slightly — some subtle diluted patches may appear. Still may not be recognizable as merle by eye alone.
Ma: Atypical merle: ~247–254 bp. Diluted or subtle merle patterning, often less patchy than classic merle. Can sometimes be missed by casual observation.
Ma+: Atypical+: ~255–264 bp. Between atypical and classic in expression.
M: Classic merle: ~265–268 bp. The standard, clearly patchy merle pattern — irregular diluted and pigmented areas within the base coat color. The most commonly recognized merle.
Mh: Harlequin merle: ~269–280 bp. Large, bold, high-contrast patches. Higher expression than classic merle. Common in the Great Dane harlequin pattern when combined with the H locus modifier.

Merle is a length, not a simple on/off

The merle allele is caused by a mobile piece of DNA — a SINE (Short Interspersed Nuclear Element) insertion — sitting within the PMEL gene. What makes merle biologically unusual is that the length of this insertion varies between individual dogs and even between generations, and the length directly determines how strongly the merle pattern is expressed.

Modern DNA tests report the actual insertion length in base pairs (bp). This is why the allele is best described as a series rather than a single variant: Mc (cryptic, ~200–230 bp) through Mh (harlequin, ~269–280 bp). The approximate length ranges above are typical; individual dogs can vary slightly.

The insertion length is genetically unstable

Unlike most genetic loci, the merle SINE insertion can shorten or lengthen when passed from parent to offspring. A classic merle (M, ~265–268 bp) parent can produce puppies with insertions in the atypical or even cryptic range — puppies that look nearly solid but carry an expressing merle allele.

This instability is the central reason why 'eyeballing' a dog for merle is dangerous. A parent that tested as classic merle can produce offspring spanning the entire spectrum — including cryptic merles with almost no visible patterning.

Double merle — the serious health emergency

When a dog inherits two expressing merle alleles (M/M — called 'double merle' or 'homozygous merle'), the same PMEL disruption that patterns the coat also profoundly affects the development of the eyes and the inner ear.

Double merles have dramatically elevated rates of: bilateral deafness (both ears), unilateral deafness (one ear), microphthalmia (abnormally small eyes), anophthalmia (absent eyes), iris abnormalities, cataracts, and other ocular defects. The severity ranges from subtle (reduced vision or high-frequency hearing loss) to complete (fully blind and/or deaf).

This is not a theoretical risk — double merles regularly appear in litters from careless pairings. The only prevention is DNA testing both parents before breeding and never pairing two merle-carrying dogs. Ethical breeders consider this an absolute rule with no exceptions.

Hidden merle — the hidden danger

A dog that is e/e (recessive red) hides merle entirely in its coat. Because eumelanin is suppressed, the merle pattern — which affects eumelanin distribution — cannot appear in the coat. The dog looks solid red, cream, or gold even if it carries M/m.

This is one of the most dangerous situations in dog breeding: a cream French Bulldog or a red Poodle that secretly carries merle, bred to a visible merle dog, can produce double-merle puppies — with parents that look as though they should be a 'safe' pairing.

Cryptic merle dogs (Mc length) that are not e/e may also show no visible pattern, appearing solid despite carrying a merle allele. Only a PMEL insertion length test (available through Embark and specialized genetics labs) reliably identifies all merle carriers.

Merle in breeds we work with

Merle is historically native to Australian Shepherds, Border Collies, Rough Collies, Shelties, Cardigan Welsh Corgis, Great Danes (harlequin system), and Dachshunds. It has been introduced to French Bulldogs, Chihuahuas, Pomeranians, some doodle lines, and other breeds through crossbreeding with merle-carrying dogs.

Wherever merle appears, the same rules apply: DNA test every dog, never pair two merle carriers, and test any red/cream dog in a line with known merle history before breeding. We test for merle insertion length on every breeding dog at Star Struck Puppies in Springfield, Georgia.

M Locus FAQ

What are the different merle lengths?

Merle is a SINE insertion whose length determines expression. The documented ranges: cryptic merle (Mc) ~200–230 bp; cryptic+ (Mc+) ~231–246 bp; atypical merle (Ma) ~247–254 bp; atypical+ (Ma+) ~255–264 bp; classic merle (M) ~265–268 bp; harlequin merle (Mh) ~269–280 bp. Longer insertions generally produce stronger, more visible merle patterning. Short insertions (cryptic) may be visually undetectable.

Why is double merle so dangerous?

A double merle (M/M) inherits two expressing merle alleles. The PMEL gene disruption that creates the mottled coat pattern also severely affects the development of the inner ear (causing deafness) and the eyes (causing microphthalmia, anophthalmia, iris defects, and blindness). Severity varies from mild hearing reduction to complete blind-and-deaf dogs. Ethical breeders NEVER pair two merle-carrying dogs.

Can merle be hidden or invisible?

Yes — in two ways. First, cryptic merle (Mc, short insertion) shows little or no visible pattern — the dog looks solid. Second, e/e (recessive red) dogs cannot express merle in the coat because there is no eumelanin for merle to dilute — a red or cream dog can be M/m with a completely solid-looking coat. Only DNA insertion length testing reveals hidden merle in both cases.

How is merle safely managed in breeding programs?

The only safe approach is: (1) DNA test every breeding dog for PMEL insertion length; (2) never pair any dog with an expressing merle allele (Mc, Mc+, Ma, Ma+, M, or Mh) with another dog that also carries any expressing merle allele; (3) also test any red/cream dogs in merle-adjacent lines, since e/e hides merle. One merle parent + one confirmed non-merle parent is always safe from a double-merle perspective.

Can the merle insertion length change between generations?

Yes. The SINE insertion is unstable and can shorten or lengthen during meiosis. A dog that tests as classic merle (M, ~265–268 bp) can produce puppies with insertions in the atypical or cryptic range — and vice versa. This is why testing parents does not guarantee predicting exact offspring allele lengths, and why any offspring of a merle parent should be tested before breeding.

Is merle in French Bulldogs ethical?

This is debated in the breed community. Merle does not naturally occur in French Bulldogs — it was introduced through crossbreeding. The ethical concerns are two-fold: (1) the health risk of double merle is the same in Frenchies as in any breed; (2) the introduction of merle into the breed gene pool requires ongoing DNA testing to prevent double-merle disasters. Breeders who DNA test every dog and never pair two merle carriers CAN produce single-merle Frenchies without producing double-merle affected puppies — but it requires strict protocols that not all breeders follow.

What is a harlequin French Bulldog or harlequin merle?

In Great Danes, harlequin is a specific M + H locus combination producing bold black patches on white. In French Bulldogs and other breeds, 'harlequin merle' is sometimes used informally to describe long-insertion merle (Mh) with high-contrast, large-patch patterning. It does not involve the Great Dane H locus. The health concerns of double merle apply equally regardless of insertion length.

How do I test my dog for merle?

PMEL insertion length testing is available through Embark DNA tests, UC Davis Veterinary Genetics Laboratory, and other specialized canine genetics labs. A swab test reports the insertion length in base pairs, classifying the dog as non-merle (m), cryptic (Mc), atypical (Ma), classic (M), or harlequin (Mh). Standard coat color panels may detect merle but not always the precise length — the length-specific test is more informative for breeding decisions.