Merle and Double Merle: Why Merle × Merle Breeding Harms Dogs

In a double merle (genotype M/M), large areas of the coat lack pigment, and malformations of the inner ear and eye appear alongside, with a clear gene-dose effect. The cause is a short SINE element inserted in reverse at the intron 10/exon 11 boundary of the PMEL gene (formerly SILV), which Clark and colleagues identified as the cause of merle in 2006.

As currently understood, PMEL builds the fibrous scaffold of the eumelanosomes where dark eumelanin pigment is deposited. The longer the insertion's poly(A) tail, the more often the gene is spliced at the wrong site and the less working PMEL is made. Because the tail length can change spontaneously in individual pigment cells, patches of full colour sit next to diluted areas. Red and yellow pigment (phaeomelanin) is not affected.

In the inner ear, degenerative changes have been described in merle dogs that reduce hearing in one or both ears. In a study of 153 merle dogs tested with BAER hearing tests (only a few of them double merles), 2.7% of single merles (Mm) were deaf in one ear and 0.9% in both. Among double merles (MM) the figures were 10% and 15%.

In the eye, double merles have been described with microphthalmia (an undersized eyeball), microcornea, a narrowed or off-centre pupil (corectopia), cataract and colobomas – gaps in the tissue that look like a keyhole in the iris. In an experimental dachshund colony, all eleven double merles examined had severe eye malformations of varying form. Milder findings, such as a missing tapetum lucidum or a poorly pigmented retina, were also seen in some single merles.

Both parents' merle status is documented by DNA test, and at most one of them carries merle. A single merle from merle × non-merle is, as a rule, a healthy dog; the risk comes almost entirely from merle × merle.

Some German breed clubs put this in writing: the German Dachshund Club forbids dapple × dapple and requires a dapple's mate to be merle-tested, the German Collie Club bans blue merle × blue merle, and the VDH breeding rules forbid any colour mating that is risky on welfare grounds.

We do not list breeders for the French Bulldog; for that breed, see the page on the short nose.

Merle is autosomal and incompletely dominant: one copy (Mm) is enough for the pattern, while two copies (MM) strengthen the effect considerably. The share of each type of puppy follows Mendel's rules on average, not in every single litter.

The table shows why merle × non-merle is fine and merle × merle is not: double merles only arise when both parents can pass on a merle copy. The breeding side – mating plans and DNA tests – is covered in our breeder guide on colour inheritance and merle.

They are versions of the same merle insertion that differ in length. The shorter it is, the weaker its effect: a cryptic merle (Mc) carries merle but looks solid-coloured. The longer it is, the more pigment is lost, up to harlequin merle (Mh), where areas of the coat turn fully white.

In 2018 Langevin and colleagues distinguished six merle alleles by length. The cut-offs are guidelines: labs measure differently, and another study grouped the same range into four categories (cryptic, dilute, standard, harlequin).

Three points matter for you. First, merle can be invisible: as a cryptic merle, or as “hidden merle” in red and yellow dogs whose coat barely shows it. Second, the length can change from one generation to the next, in 4.6% of dogs in one study. Third, mosaicism is common; Langevin found it in 16.6% of the dogs tested.

The Great Dane's harlequin is related but not the same as harlequin merle: it arises from one classic merle copy plus a second gene variant (PSMB7).

Not explicitly, but Section 11b of the German Animal Welfare Act (TierSchG) can be applied to it: merle is not named in the statute itself, yet the rule prohibits breeding vertebrates when offspring are expected to suffer hereditary pain, suffering or damage because body parts or organs are missing, unfit or altered. The authorities can also order animals to be sterilised if their offspring are expected to show such defects.

How Section 11b should be applied is set out in the “Expert opinion on the interpretation of Section 11b of the Animal Welfare Act (ban on extreme breeding)”, usually dated 1999, which authorities use as their reference. It is an expert interpretation aid, not law, and predates today's merle DNA tests. It lists the merle syndrome as a specific example and recommends a breeding ban for double merles and for merle × merle matings (Mm × Mm). Because sensory changes have also been described in single merles, it goes further and advises against breeding with the merle gene at all. Breeding rules such as those of the German Dachshund and Collie clubs still allow merle × non-merle.

Since 1 January 2022, Section 10 of the German Dog Welfare Ordinance (Tierschutz-Hundeverordnung) has also banned showing dogs with such hereditary traits at exhibitions and other events where dogs are judged. Whether an individual dog falls under it is assessed case by case. Owning a double merle is not prohibited; the bans cover breeding and exhibiting.

Switzerland is more specific: under Art. 9 of the federal breeding ordinance (TSchZV), matings in which a loss of the senses, namely blindness or deafness, cannot be ruled out are banned. The rule can be read as covering merle × merle, although merle is not named in it and cases are assessed individually.