UC Davis School of Veterinary Medicine Veterinary Genetics Laboratory
Merle

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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. Blue and partially blue eyes are typically seen with merle, and merle dogs often have a wide range of auditory and ophthalmologic defects.  Dogs with 2 copies of merle (called double merle) are primarily white and can have multiple abnormalities of skeletal, cardiac and reproductive systems, thus breedings between 2 merle dogs are discouraged to avoid producing double merle offspring. Merle is governed by a SINE insertion in the Pmel17 or Silver (SILV) gene. In the horse, a mutation in this gene results in eumelanin pigment dilution and is also associated with eye abnormalities.

Merle only dilutes eumelanin (black) pigment; dogs with an MC1R ee genotype (recessive red) have no black pigment, thus do not express merle but can produce merle offspring. There are 3 alleles (variants) for merle: merle (M allele, SINE with longer poly-A tail), cryptic merle (Mc allele, SINE with shorter poly-A tail) and non-merle (N allele, no SINE insertion). Dogs with cryptic merle (also called phantom or ghost merle), typically display little to no merling and some may be misclassified as non-merles. Inheritance of merle is genetically unstable for both M and Mc alleles. During DNA replication and cell division, M may occasionally undergo poly-A tail reduction to produce Mc (germline rate of 3-4%) while Mc may undergo expansion and revert to M. Because of the complexities of merle inheritance, and potential health concerns, DNA testing is recommended to establish the genetic make-up of dogs for the merle gene for those breeds where this color dilution pattern is present.

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

Results are reported as:

M/M 2 copies of merle are present (double merle)
M/Mc 1 copy of merle and 1 copy of cryptic merle are present
M/N 1 copy of merle is present
Mc/Mc 2 copies of cryptic merle are present
Mc/N 1 copy of cryptic merle is present
N/N No copies of merle or cryptic merle are present

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 U S A. 103(5):1376-1381 (2006).

Kaelin CB, Barsh GS. Genetics of pigmentation in dogs and cats. Annu. Rev. Anim. Biosci. 1:16.1-16.32 (2013).

Veterinary Genetics Laboratory, Tel 530-752-2211, Email VGL