There are several merle dog breeds where the pattern is commonly found and accepted as a breed standard, including:
Research shows that the gene responsible for merle in dogs is the same in every breed, indicating that it is an ancient mutation that predates the formation of dog breeds. It is unlikely to have arisen independently in different breeds.
What Gene Causes Merle in Dogs’ Coat?
The merle coat color lies in basic genetics, where there is a dominant and recessive trait to produce those gorgeous splotches. This is the merle allele (M) and one copy of the non-merle allele (m). The merle (M) allele is a semi-dominant gene, meaning it only takes one copy of the M allele to produce a merle. So, every merle dog has one copy of the merle allele (M) and one copy of the non-merle allele (m), meaning every merle dog has an Mm genotype, and, therefore, every non-merle dog has an mm genotype.
The merle allele was first discovered at Texas A&M University’s College of Veterinary Medicine in 2006. The merle coat color is governed by a type of mutation called a SINE insertion in the SILV (also called PMEL17) gene.
What about dogs with two merle alleles (MM)? These “double merles” (or “double-dapples”) don’t look like merles. They usually have much more white on them—and some can be almost pure white. The fully pigmented splotches are much smaller, and the background color is much whiter.
Why Do Some Dogs Have More Merle Than Others?
You may notice that some dogs only have the merle pattern on their face, for example, while others show splotches on their entire body. There’s some additional science behind that. The SILV gene involved with merle in dogs is responsible for producing a matrix that essentially holds the pigment in place. In a non-merle dog, the matrix is completely formed and pigment stays put. But if one SILV allele has this insertion of extra genetic material in it, the matrix has holes in it. Pigment granules escape from the holes, leaving a faded coat color.
However, the length of this genetic insertion is not very stable, and as cells divide during embryogenesis, which is the process of the development of an embryo, it may shrink or expand. In some embryonic cells, it shrinks to the point of being nearly normal, and the matrix these cells produce is almost complete. During development, cells derived from these near-normal embryonic cells give rise to patches of the fully pigmented coat.
Thus, merles are a mosaic of copies derived from cells with various degrees of “leaky” matrixes and normal matrixes. The size of each patch depends on how early in embryogenesis the insertion size mutated, with larger patches descended from earlier mutation events.
Merle is a complicated and fascinating color pattern—both in appearance and in genetics. That’s why dog breeders and geneticists alike consider merle in dogs beautiful.
