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UC Davis now offers the MERLE Test!  

We would like to thank Alesia Dixon at and Mon Cheri Frenchies at

for providing us with this material

Merle is an incompletely dominant coat color pattern characterized by irregularly shaped patches of diluted pigment and solid color. Breeds with merle include but are not limited to: Shetland Sheepdog, Collie, Border Collie, Australian Shepherd, Cardigan Welsh Corgi, Catahoula Leopard Dog, Dachshund, Great Dane, Bergamasco Sheepdog and Pyrenean Shepherd.

The Veterinary Genetics Laboratory is licensed to offer the merle test.

Results are reported as

M/M2 copies of merle are present (double merle)
M/Mc1 copy of merle and 1 copy of cryptic merle are present
M/N1 copy of merle is present
Mc/Mc2 copies of cryptic merle are present
Mc/N1 copy of cryptic merle is present
N/NNo copies of merle or cryptic merle are present

Merle can be hidden by other genes,  and patterns, sometimes for long periods of time. This is why the merle gene can & has ALWAYS been in EVERY breed and is backed up by scientific fact.

Breeding Information

The M locus is the home of the merle allele. Merle is dominant, and so denoted by the capital letter M. Non-merle is recessive, and denoted by m. Merle is interesting because all normal merles are heterozygous (Mm). A homozygous merle is actually a double merle.

Merle affects only eumelanin. That means that any black, liver, blue or lilac in the coat, eyes or nose will be merled, whether it's the whole of the body, a mask on a sable, shading, brindle stripes, or even a saddle. Phaeomelanin (red) is not affected at all and will appear as normal.
Most Black based Merle dogs are called "blue merles" because of the bluish color between the patches in their coat. This is a widely-used term but is actually misleading. Technically they should be "black merles". Their nose pigment is black and their eyes are brown or blue. They are able to make normal eumelanin in their coat, so their patches are black. If they didn't have the merle gene, they would be solid black. "Blue merle" is misleading because it seems to say that these dogs have blue pigment (dd acting on black), when in fact they have black.

The random coat dilution caused by merle also affects the eyes and nose. The eyes may be all or partly blue, and the nose may be all or partly pink. Not all merles have blue eyes or pink noses though, and merles with heavy dark patching are more likely to have normal eye and nose pigment.
Due to the unstable and variable nature of the merle gene, sometimes merles have patches that are only partially diluted, and are between the base and the patch color. These are known as dilute spots, and they may sometimes appear brownish. Dogs with very extensive dilute and/or brown patching are likely to be tweed merles.

We talk about coat color being "diluted" in a merle, but note that dilute spots don't have anything at all to do with the Dilution gene (d), and are just a normal variation of the merle pattern.

Merle acts on the black pigment in the iris of the eye just as it does on the coat, so merle dogs often have part or all of the eye blue. (This does not affect their vision, though since it happens to some extent in the retina as well it may make it harder to diagnose certain eye problems.)
There are always two copies of a gene, alike or different, in any dog. If we call the merle gene M and the non-merle gene m, any given dog can be mm, Mm or MM. The mm dog is the normal.  The Mm dog is a blue merle, chocolate merle, lilac merle, black merle, or sable merle, depending on what color it would have been without the merling gene. An MM dog, often called a double merle or a homozygous merle, will be mostly white and sometimes can be associated with deafness or blindness.

Breeding merle to full color will produce one half full color and one half merles, but no defective whites. The merle to full color breeding, then, produces just as many merles as does the merle to merle breeding, and without the danger of defective puppies. The safe breeding for a merle, then, is to a non-merle mate. This breeding should produce all healthy puppies, and about half will be merles.

Sable merles are no more likely to have health problems than any other color.  The real argument against sable merles is that they may be mistaken for normal sables. If two such sable merles were mated together, the resulting litter could contain defective whites. What a shock for the breeder that may produce lethal whites.

Very few breeders have been lucky enough to get high quality homozygous (double) merles that are not too severely affected to breed - but it definitely takes a lot of luck and really top quality black merles to start with. Merle to merle breedings are only for the very experienced breeder who knows his/her lines and what they will produce, and it has probably produced more heartbreaks than good homozygous merles, even for the experienced breeder.
There is no such thing as a sable, blue, chocolate, lilac, or black merle gene. There is only a merle gene. Merle is a dilution gene, that is, it lightens whatever the coat color would otherwise have been. The lightening is not spread evenly over the coat, but leaves patches of undiluted color scattered over the dog's body. Also, the lightening seems to work primarily on the black pigment in the coat, so any tan on the face stays even.

Cryptic Merles

Cryptic merle (phantom merle) are dogs that show only very slight merle coloration and in some cases it is not visible at all. The dog can have only small patches of merle, for example, at the end of tail or ear or the merle coloration can be concealed by white markings.

These dogs carry the shorter version of the merle gene, sometimes one copy and sometimes two copies. Unlike regular merle dogs, in the cryptic merle dogs no serious health problems connected with the regular (not shortened) merle allele have been described. They apparently have no eye or hearing problems. Dogs with two copies of cryptic merle gene (Mc/Mc genotype) or dogs with one cryptic merle copy and one regular merle copy (M/Mc genotype) have no health problems. The correct description of cryptic merle is a problem when registering the dog. These dogs appear like normal colored and are usually registered as non-merle dogs since not every breed registry color code includes the merle pattern as a choice.

Frequent mistakes: Excessive white markings in puppies from a tri-to-merle cross are not an indication that the puppy is a cryptic merle. The genetics of excessive white markings is completely different and have nothing to do with merle gene.

In breeding, a cryptic merle can be mated only with non-merle dogs (like dogs with regular merle allele). When crossed, the cryptic allele may expand again to regular non-shortened merle allele.  When mating a cryptic merle (Mc/N) with a non-merle (N/N) you can find puppies with the following genotypes: Cryptic merle/non-merle (Mc/N), Merle/ non-merle (M/N), non-merle/non-merle (N/N).

Hidden Merles

Hidden merles are merle dogs who do not exhibit the merle pattern because their coat color does not show the pattern. Merling is not normally shown in red, gold, fawn and cream coat colors.  The hidden merle can be distinguished only by a genetic test.

Recessive red and merle can be a dangerous combo, simply because you may not know that a recessive red dog is a merle. The two genes occur together in a number of breeds, including Pomeranians and Chihuahuas, and in such breeds it's advisable to never breed a clear red dog (with any merle in its ancestry) to a merle, due to the risk of accidentally breeding double merles. Getting your dogs tested for Merle eliminates this problem.

Clear sable can "hide" merle almost as effectively as recessive red. A clear sable is one with no dark (eumelanin) hairs in its coat, and clear sables may be almost indistinguishable from recessive reds. For this reason, care should be taken when breeding any solid red dogs in breeds where merle is present.


Genetic testing for merle gene is highly recommended in order to avoid severe health problems that may occur when merle, cryptic merle and hidden merle dogs are crossed incorrectly or in case of risky breeding. The genetic test reveals the merle, the hidden merle and the cryptic merle variants.

All puppies that appear as "non-merle" at first sight have to be properly checked for any less visible merle patches. If such patch is found, it is not a non-merle, but a cryptic merle. In particular, the small merle patch can disappear in long or thick hair. Later, such dog can be mated with another cryptic/hidden merle dog unintentionally. So the breeders can run unintentional risks of producing double merle dogs.

Merle Modifiers

A merle modifier is a gene that, when inherited along with merle, will affect the way the merle pattern appears. A dog with a merle modifier but no merle gene will not be affected at all. It's thought that merle modifiers are inherited separately from merle and appear on their own locii. Merle Modifiers: TweedHarlequin, & Pseudo Harlequin

Merle Modifiers

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