Understanding merle: inheritance, genotypes and what matters in a mating

What merle is genetically

Merle is caused by an insertion in the PMEL17 gene, also known as SILV. A stretch of mobile DNA, a so-called SINE insertion, lodges itself in that gene and disrupts the production of eumelanin, the dark pigment. The result is the familiar marbled pattern: fully pigmented areas sitting next to diluted patches.

Crucially, the dilution affects dark pigment only. Red and copper markings stay as they are, and so do white areas. That is why a blue merle shows marbling in the black coat while the copper points remain clearly defined.

Merle is incompletely dominant. A single copy is enough for the pattern to show. Two copies produce a far stronger dilution — and considerable health risks.

Allele lengths and what they mean

The inserted sequence is not the same length in every dog. That length, measured in base pairs, is what the merle variants are classified by. The longer the insertion, the stronger its effect on the coat.

This classification is a guide, not a guarantee. Dogs with the same allele length can look different, and the class says nothing about the size or distribution of the patches.

The insertion is unstable in length

One feature often overlooked in breeding plans: the length of the insertion can change as it is passed on. A dog with classic merle can produce offspring in which the allele has shortened — all the way down to cryptic merle, which no longer shows in the coat. The appearance of a litter therefore cannot be predicted precisely.

Why merle to merle is ruled out

The link is well documented and not in dispute. Reputable breed clubs exclude merle to merle pairings, and even without a club rule this is simply something a responsible breeder does not do.

In Germany such a mating may also fall under the prohibition of torture breeding. The Animal Welfare Act forbids breeding vertebrates where offspring are born with missing or non-functional body parts or organs for hereditary reasons.

Cryptic merle: the invisible trap

The greatest practical risk does not come from obviously merle dogs, but from the inconspicuous ones. A dog carrying cryptic merle looks like an ordinary black tri or red tri. Nothing gives it away — and it passes merle on regardless.

Mate such a dog with a merle partner and you create exactly the combination you set out to avoid. The puppies are double merle, and nobody could have known in advance.

How to read a result

A merle report names the two alleles the dog carries. A few typical examples:

• m/m — no merle. The dog carries nothing and can safely be paired with a merle partner.
• M/m — one copy of classic merle, one wild type copy. Half the offspring inherit merle.
• Mc/m — cryptic merle. The dog does not look merle but passes the variant on.
• Mc/M — two merle variants, no wild type copy. Every offspring inherits one of the two.

The last case deserves particular attention. Such a dog passes merle on in every case, not just half the time. A pairing with a merle partner is therefore entirely ruled out, not merely risky.

What else belongs in the picture

Merle is not the only factor in colour planning. Anyone preparing a mating is well advised to look at the other loci too: the A locus for base pattern, the B locus for brown, the D locus for dilution, the S locus for white spotting. The S locus matters in particular, because excessive white carries risks of its own.

Independently of colour, the health-related genetic tests belong in the picture — MDR1 above all, together with CEA, PRA, HSF4, DM and NAD. Which examinations should be on the table before a stud decision is covered in the stud dog checklist.