The Merle Reference Centre

Merle is caused by a SINE insertion in the PMEL gene. What matters is not whether the insert is present but how long it is. Labs that offer length-based testing report that length in base pairs, and the number places the dog in one of seven classes.

The right-hand column is the phenotype when the other parent contributes a plain non-merle allele. Two merle alleles together behave differently, which is what the calculator models.

The single most useful thing a breeder can know about merle is where the visible threshold sits. It is between Ma and Ma+, not at the bottom of the scale.

• Mc, Mc+, and even a single Ma produce no visible pattern at all. The dog looks solid.
• Two Ma together begin to express, weakly, with no body white.
• Ma+ is a genuine threshold. Near the top of its range it shows a muted, often diluted pattern. Lower down it may show nothing.
• M and Mh express clearly.

This is why a solid-looking poodle can produce merle puppies, and why no coat inspection can rule merle out. Only a test can.

Most labs still report merle as a simple presence or absence, typically as M/mor “merle detected”. That answer is not wrong, but it collapses seven classes into two categories and discards the number that determines risk.

A binary result cannot distinguish a cryptic Mc from a harlequin Mh, even though those two dogs carry entirely different consequences for a pairing. If you are breeding merle deliberately, ask your lab whether they report the base-pair length, not just the presence.

Because reported values vary between labs by roughly two base pairs, a result sitting near a class boundary should be read as the more cautious, longer class. The calculator applies that tolerance automatically when you enter an exact value.

The informal vocabulary around invisible merles is a mess, and in poodles one term is actively harmful. Elsewhere in the dog world “phantom merle” is used loosely to mean a merle you cannot see. In poodles, phantom already means the tan-point pattern. Using it for merle collides with an established poodle term and produces real confusion in pedigrees and sales listings.

Say cryptic merle when you mean an insert too short to express, and hidden merle when you mean a merle you simply cannot observe, whatever the reason. Both are unambiguous.

A dog that is ee at the E locus produces no eumelanin in its coat. Merle patterns eumelanin, so on an ee dog there is nothing for it to pattern. The coat shows no merle whatsoever.

The allele is completely unaffected. The dog is a merle, passes it on at the normal rate, and can produce a double merle if bred to another merle. This is one of the most common routes to an accidental double merle litter, because both parents looked entirely clear.

The poly-A tail of the insert is unstable. It shortens readily during cell division and has never been confirmed to lengthen. When that happens in body cells you get a somatic mosaic, often visible as tweed. When it happens in the reproductive line you get a germline mosaic.

A germline mosaic carries a second, shorter merle allele in its sperm or eggs that may never appear on its own DNA report. It can therefore produce a puppy with an allele neither parent was known to carry.

Roughly ~18% (about 1 in 5–6) of merles are mosaic, and about ~38% have a shortened Mc minor allele. Those are population figures, not per-litter odds, and that distinction matters: there is no honest per-cross probability for mosaicism. Any tool that offers you one is inventing it. The calculator treats it as a caution layer rather than a number.

When two expressing merle alleles meet, pigment is deleted across the body rather than patterned. Because the same pigment cells are involved in the inner ear and in eye development, extensive pigment loss carries a genuine risk of deafness and of ocular defects.

BAER testing (brainstem auditory evoked response) is the only objective way to establish whether a dog hears, and it detects unilateral deafness that behavioural observation reliably misses. A dog deaf in one ear usually behaves entirely normally. If a pairing could produce extensive white, BAER testing the resulting puppies is the responsible step, and an ophthalmologic examination is worth discussing with your veterinarian.

Not every merle pairing carries this risk. The calculator separates the genuinely critical combinations from the ones that merely need care, so the warning stays meaningful.

Cryptic merle

A merle whose insert is short enough that it produces no visible pattern. It is still a merle and still passes the allele on. Mc and Mc+ are cryptic, and a single Ma behaves the same way.

Hidden merle

An informal term for any merle you cannot see, whether because the insert is cryptic, because the dog is ee, or because white markings cover the patterned areas. Not a genotype, just a description of what you cannot observe.

Phantom merle

A confusing term best avoided in poodles. Elsewhere it is used loosely to mean cryptic merle, but in poodles "phantom" already means the tan-point pattern. Say cryptic merle instead, and the ambiguity disappears.

Ghost merle

Another informal synonym for cryptic merle. Same advice: name the allele class instead.

Atypical merle

The Ma and Ma+ classes. Historically described as odd or washed-out merles, now understood as a length effect rather than a separate gene.

Harlequin (Mh)

The longest class, 269 bp and above. Expresses as mostly solid pigment with random merle patches and can carry extended white. The most unstable class, and every Mh pairing warrants expert review.

Double merle

A dog carrying two expressing merle alleles. Pigment is deleted to white across the body, with a real risk of deafness and blindness. Never intentionally produced.

Tweed

A patchwork of differently toned areas within a merle coat, produced when the insert shortens in some cell lineages during development.

Somatic mosaic

A dog in whom the merle insert shortened in some body cells after conception. The coat may show more than one merle expression, but only the germline determines what is passed on.

Germline mosaic

A dog carrying a second, shorter merle allele in its reproductive cells. It can produce puppies with an allele that never appeared on its own DNA report, which is why no calculator can give an exact per-cross probability.

Minor allele

The shorter, secondary merle allele found in a mosaic dog, alongside the primary reported one.

SINE insertion

The short interspersed nuclear element inserted into the PMEL gene that causes merle. Its length, specifically the length of its poly-A tail, determines the class.

Poly-A tail

The unstable run of adenines at the end of the insert. It shortens readily and is why merle length changes between generations, and why it has never been confirmed to lengthen.

• Whether a specific dog is a merle. Only a DNA test can establish that.
• How much white a given puppy will carry. The amount of white is influenced by modifiers that are not currently testable.
• Whether a mosaic parent will pass its hidden minor allele in any particular mating.
• Whether a dog hears. That requires BAER testing, not observation and not a pedigree.

The length-allele framework follows the work of M. Langevin and colleagues on PMEL SINE insertion length. The pairing risk bands used in the calculator are the author’s own. This page is educational and does not replace laboratory testing or veterinary advice.