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. Note that "black" as used here includes liver or chocolate. Merle is a distinguishing marking of several breeds. The merle gene also plays a part in producing harlequin. The most recognizable is the blue or red merle, but chocolate merle is also possible (see photos below).
Chocolate Merle Blue Merle
Merle refers to the pattern in the coat and is not a color as such. The white and gray patterns that appear on a black make them appear to have a blueish cast. These are called blue merles. Merle is a color combination in dogs' coats. It is a solid base color (usually red/brown or black) with lighter blue/gray or reddish patches, which gives a mottled or uneven speckled effect. Although most breeds that can have merle coats also typically have white markings (such as around the neck, under the belly, and so on), and often tan points (typically between the white and the darker parts of the coat), these are separate colors from the merle; some dogs do appear completely merled with no white or tan markings. Merle can also alter other colors and patterns besides the usual red or black. These combinations such as Brindle Merle or Liver Merle.
In addition to altering base coat color, merle also modifies eye color and coloring on the nose and paw pads. The merle gene modifies the dark pigment in the eyes, occasionally changing dark eyes to blue, or part of the eye to be colored blue. Since merle causes random modifications, however, both dark-eyed, blue-eyed, and odd-colored eyes are possible. Color on paw pads and nose may be mottled pink and black (see photo).
Mottled paw pads on chocolate merle pup
Merle is actually a heterozygote of an incompletely dominant gene. If two such dogs are mated, on the average one quarter of the puppies will be "double merles". A phantom merle is one with such small patches of merle or none at all that it appears to be a non-merle. In America, a dog with the phantom merle coloring is described as being "cryptic for merle."
Where does the merle gene come from and how it got in the poodle gene pool?
I get this question all the time. I did not create the merle gene. I have done a lot of study of the merle gene in the poodle and pomeranian gene pool and believe that it was not caused by introducing another breed into the gene pool but, in fact, it is a mutation that occurred naturally.
Merle is probably the most mis-understood and hotly debated dilution gene in the poodle. Many believe it must be a recessive gene because it doesn't affect the appearance of red, apricot and red or diluted colors such as blues and silvers. In addition, it can be hard to see or not evident at all on black and brown dogs (cryptic). Not only is its appearance illusive to breeders but it has confused scientists and it is still not completely understood.
The merle pattern has been studied many times and the last study released in 2006 emphatically stated that merle was caused by a mutation in the gene called SILV or PMEL17. The research was believed to be so solid that tests were offered to breeders to identify Merle in their dogs. However, as of March 2009 the company who owned the patent for the test, Idexx, stopped offering tests as they discovered the scientists identified the wrong gene. SILV is not responsible for merle. The gene they decided was merle (SILV) actually turned out to be for a form of piebaldism- not merle.
We have been saying for years that it can be difficult to tell by sight alone the difference between double merle dogs and piebald dogs, and evidently genetic researchers could not either. Any study or research quoting SILV or referring to the Idexx website is outdated and incorrect!!
What we know for certain is that Merle was likely a recessive gene at one point as it is still in many breeds. Why it now expresses as a dominant in some breeds is unknown. (still to be studied).
Theories of the merle gene:
One theory is that piebaldism may be issue when connected to merle- or it may be piebaldism itself that is the issue.
Another theory is "The Misidentification Theory." It is believed that merles have been in the breed and that merle is a "natural" occuring pattern in the breed. However, it is said - rightly so - that merles have often been misidentified as other colors.
Then there is the "The Cryptic Merle" theory. This theory is actually a subcategory of the Misidentification Theory. Merle is what is called a "modifier." That means it "modifies" (i.e. changes) a coat color that has already been determined by genes at other locations. If merle acts on a black coat, the result is blue merle. If it acts on a chocolate coat, the result is red merle. And so on.
The final theory we have is "The Mutation Theory." According to a recent study, merle is a very old mutation said to be present in the genome of the common ancestor of ALL dog breeds - to include the poodle. As a result, while merle is an autosomal incomplete dominant, it can still mutate into a breed not thought to have merle. The most recent find of merle in a supposed non-merle carrying breed is the poodle.
There are a lot of cross bred Merle Poodles mixes who are NOT purebred dogs- Most cross bred poodle mixes shed and are not registered with an official kennel club (such as AKC). Although they may be cute, they do not have the allergy friendly qualities of the purebred poodle.
This document is research in progress and through experience in breeding merles. Most of the information was obtained from the world wide web.