The M Locus (Merle) coat color test reliably determines if a dog carries the M (merle insertion variant) Allele of the PMEL gene. If present, this test will also determine the approximate size (+/- 1 base pair) of the M allele(s).
Merle is inherited in a dominant fashion, meaning that only one copy of an M allele is necessary for a dog to display some variation of the merle coat color/pattern, which is marked by random dilution of eumelanin (black pigment) leaving patches of normal coat color within areas of diluted pigmentation. The M allele impacts canine coat color by altering the expression of the PMEL gene which is primarily located in eumelanosomes that produce and store eumelanin (black pigment). Therefore, the M variant of PMEL typically affects only eumelanin pigmented areas and not phaeomelanin (yellow/red) pigmented areas. The patchiness of the merle coat color is thought to be due to the unstable nature of the M variant which results in patches of skin cells with different sizes of M alleles that may express the coat color or pattern differently. The size of the of the M allele(s) present in a dog have been associated with different merle coat colors/patterns. In addition, two copies of the M allele large enough to produce the merle coat color are thought to cause the death of skin melanocytes, retinal pigment cells, and melanocytes of the inner ear which can result in significant white areas of the coat, eye abnormalities and deafness.