M Locus (Merle)

Other Names: M locus, merle coat color, merle locus
Affected Genes: PMEL
Inheritance: Autosomal Dominant
Mutation: Complex Rearrangement
Breed(s): All Breeds
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Testing Summary

The M Locus (Merle) coat color test reliably determines if a dog has one of the following genotypes at the M locus:

M/M

This dog carries two copies of M which produces a widespread merle coat color pattern that is mostly white with small patches of normal color. The merle coat color results from random dilution of eumelanin (black pigment) leaving patches of normal coat color within areas of diluted pigmentation. This dog is also considered an affected, "double merle" because M/M dogs occasionally exhibit deafness and ocular defects. This dog will pass on M to 100% of its offspring.

Interpretation: Affected, double merle
m/M

This dog carries one copy of M and one copy of m which results in a mild to moderate merle coat color pattern that consists of random patches of normal coat color surrounded by regions of diluted eumelanin (black pigment). This dog will pass on M to 50% of its offspring and m to 50% of its offspring.

Interpretation: Merle
m/m

This dog carries two copies of m and does not have a merle coat color. This dog will pass on m to 100% of its offspring.

Interpretation: Non merle


Detailed Summary

The merle (M) Locus 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 can revert back to the normal m variant in some cells during development giving rise to patches of normal eumelanin producing cells. Two copies of M are thought to cause the death of skin melanocytes, retinal pigment cells, and melanocytes of the inner ear which can result in white areas of the coat, ocular defects and deafness.


Testing Tips

The unstable nature of the M variant, as described above, is also thought to be the basis for what is known as "cryptic" merle dogs that display very little to no merle coat color but can produce offspring with the typical merle coat color pattern. Genetic testing of the PMEL gene will determine whether a dog is a genetic Carrier of merle coat color. However, in order to avoid producing pups with ocular and auditory problems, breeding of known carriers to each other is not recommended as 25% of pups will be "double merles".


References

  • Clark LA, Wahl JM, Rees CA, Murphy KE. Retrotransposon insertion in SILV is responsible for merle patterning of the domestic dog. Proc Natl Acad Sci USA. 2006 Jan 31; 103(5):1376-81. [PubMed: 16407134]