Advanced Tool
Advanced Merle Calculator
Merle is a length continuum, not an on/off switch. This models all seven M-locus alleles and places every pairing on the risk matrix — Safe through High risk.
Parent A
as a dog: SafeClassic merle pattern. Non-merle wild type. No pattern, nothing deleted.
Parent B
as a dog: SafeClassic merle pattern. Non-merle wild type. No pattern, nothing deleted.
Critical: double-merle risk
Each puppy has a 25% chance of being a critical double merle — a real risk of deafness and blindness. This pairing is never intentionally produced.
Offspring M-locus outcomes
Take this pairing with you — free PDF report
Both parents, every outcome and risk band, the complete pairing matrix, the mosaic caution, and a report ID. Free — just tell us where to send updates.
Why the percentages above are a floor, not a guarantee
The merle insertion is unstable: its tail shortens over time, and it is never confirmed to lengthen. Two consequences the Punnett above cannot show, because they are not predictable:
Mosaics (in the individual). If the insertion shortens in body cells early in development, the dog becomes a patchwork carrying more than one M-locus length. Which areas are affected is tissue-specific and cannot be predicted — it is read from testing, not calculated. In this cross, a puppy that inherits M can shorten it in some cells — the insertion collapses toward the shorter classes, most often to Mc — and would then test as a mosaic carrying M together with a shorter allele. That is a mosaic of two lengths in one dog, not a pairing inherited from the parents.
Germline surprises (in the offspring). A merle parent can carry a hidden, usually shorter minor allele in its eggs or sperm. So a dog testing for a long allele can throw shorter — even non-merle — puppies, and a dog that looks plain can throw a merle. It is passing an allele it already carries but that a single test may miss, not making a new length at conception. The shift is toward shorter and safer, never confirmed the other way.
For scale, not for this litter: roughly ~18% (about 1 in 5–6) of merles are mosaic, and ~38% have a shortened Mc minor allele. These are population figures, not odds for your cross — no one has a per-litter probability for this, and this tool will not invent one.
The safeguard is testing, not prediction: test breeding-prospect offspring, and test semen on merle sires, since a mosaic minor allele will not show in the headline genotype.
Risk bands
- SafeNo pigment deleted at the M locus.
- Low riskConditionally safe cryptic pairings.
- Medium riskAcceptable in controlled, fully documented programs.
- Elevated riskManaged, not avoided — always with the lowest partner allele.
- High riskNever intentionally produced.
Allele pairing phenotype reference
Every homozygous and heterozygous M-locus pairing, and what a dog with it looks like. A visible pattern only begins with two expressing alleles: the shorter alleles (Mc, Mc+) never show, a single Ma or Ma+ stays non-expressing on its own, and it takes two Ma together — or Ma+ paired with another expressing allele — to pattern a coat. The illustrations are indicative, not photographs.
Non-merle. A solid coat, with no merle pattern and none to pass on.
No visible merle detected — non-expressing (cryptic). Looks solid, at most a little coat-colour dilution, but has the insertion and passes it on.
No visible merle detected — non-expressing (cryptic). Looks solid, at most a little coat-colour dilution, but has the insertion and passes it on.
No visible merle detected — non-expressing (cryptic). Looks solid, at most a little coat-colour dilution, but has the insertion and passes it on.
No visible merle detected — non-expressing (cryptic). Looks solid, at most a little coat-colour dilution, but has the insertion and passes it on.
No visible merle detected — non-expressing (cryptic). Looks solid, at most a little coat-colour dilution, but has the insertion and passes it on.
No visible merle detected — non-expressing (cryptic). Looks solid, at most a little coat-colour dilution, but has the insertion and passes it on.
No visible merle detected — non-expressing (cryptic). Looks solid, at most a little coat-colour dilution, but has the insertion and passes it on.
No visible merle detected — non-expressing (cryptic). Looks solid, at most a little coat-colour dilution, but has the insertion and passes it on.
Non-expressing on its own — a single Ma+ shows no merle pattern and deletes no pigment to white. At most it may add a little greying (slightly more without the dilute gene), and even that is minimal. Whether it does anything at all depends entirely on the exact length of the Ma+ allele.
Non-expressing on its own — a single Ma+ shows no merle pattern and deletes no pigment to white. At most it may add a little greying (slightly more without the dilute gene), and even that is minimal. Whether it does anything at all depends entirely on the exact length of the Ma+ allele.
Non-expressing on its own — a single Ma+ shows no merle pattern and deletes no pigment to white. At most it may add a little greying (slightly more without the dilute gene), and even that is minimal. Whether it does anything at all depends entirely on the exact length of the Ma+ allele.
Single merle — the classic mottled pattern over the coat; little to no body white.
Harlequin-type single merle — mostly solid with random merle patches, and can have some white (collar, legs, belly).
Single merle — the classic mottled pattern over the coat; little to no body white.
Two atypical alleles (Ma/Ma) — a weak, often diluted merle pattern with no body white.
Two merle alleles expressing together — some pigment is deleted to white, a borderline health risk. Have it expert-reviewed before breeding.
Two merle alleles expressing together — some pigment is deleted to white, a borderline health risk. Have it expert-reviewed before breeding.
Two merle alleles expressing together — some pigment is deleted to white, a borderline health risk. Have it expert-reviewed before breeding. With Ma+ in the pair (Ma+/Ma+ or Ma+/M), how much actually shows depends entirely on the exact length of the Ma+ allele.
Two merle alleles expressing together — some pigment is deleted to white, a borderline health risk. Have it expert-reviewed before breeding.
Two merle alleles expressing together — some pigment is deleted to white, a borderline health risk. Have it expert-reviewed before breeding. With Ma+ in the pair (Ma+/Ma+ or Ma+/M), how much actually shows depends entirely on the exact length of the Ma+ allele.
Two merle alleles expressing together — some pigment is deleted to white, a borderline health risk. Have it expert-reviewed before breeding. With Ma+ in the pair (Ma+/Ma+ or Ma+/M), how much actually shows depends entirely on the exact length of the Ma+ allele.
Critical double merle — extensive white from pigment loss, with a real risk of deafness and blindness. Never intentionally produced.
Critical double merle — extensive white from pigment loss, with a real risk of deafness and blindness. Never intentionally produced.
Critical double merle — extensive white from pigment loss, with a real risk of deafness and blindness. Never intentionally produced.
Critical double merle — extensive white from pigment loss, with a real risk of deafness and blindness. Never intentionally produced.
Critical double merle — extensive white from pigment loss, with a real risk of deafness and blindness. Never intentionally produced.
Critical double merle — extensive white from pigment loss, with a real risk of deafness and blindness. Never intentionally produced.
The class is the insertion length, and the boundaries are reporting conventions, not biological walls — a 254 bp and a 255 bp allele are neighbours, not opposites. Labs also read the same allele about ±2 bpapart, so a value near a boundary can genuinely belong to either side. Enter your lab’s exact value above and the tool flags when that happens; treat any near-boundary result as the more cautious class. A risk band is the start of the decision, not the decision. See the merle article for the full picture. Educational use only.