E locus (Extension)
E locus (extension) is also known as the melanocortin 1 receptor (MC1R). MC1R was the first gene studied using molecular approaches in dogs. The E locus has three known alleles in dogs: Em > E > e. The wild-type allele, E, encodes a functional MC1R that allows for expression of the Agouti and K locus alleles. Coat color phenotypes which show a mixture of pheomelanin and eumelanin in different regions of the body are generally caused by regional expression of Agouti or genetically unstable MC1R alleles associated with somatic mosaicism. Black mask is a characteristic pattern in which red, yellow, tan, fawn, or brindle dogs exhibit a melanistic muzzle which may extend up onto the ears. Melanistic mask is inherited in several breeds as an autosomal dominant trait, and appears to be a fixed trait in a few breeds of dogs.