Melanistic Mask – EM Locus
The dominant Em allele of MC1R is responsible for the localized distribution of eumelanin on the muzzle that resembles a darkened mask in a pheomelanic (ay) background. The melanistic mask caused by one copy of this allele is only visible on dogs that are fawn or brindle. The mask can be black, liver, blue or isabella on the muzzle and sometimes the ears. Dogs that are solid black, brown, or blue do not have a mask that is distinguishable from their body colour. However, dogs that fade to grey with age do show their mask for a time. Similarly, dogs that have white muzzles do not produce melanin in that area of the body and so they may not exhibit a mask, even if they carry this allele.
Schmutz, S.M., Berryere, T.G., Ellinwood, N.M., Kerns, J.A., and Barsh, G.S. (2003). MC1R Studies in Dogs With Melanistic Mask or Brindle Patterns. J Hered 94, 69–73.