MC1R, also known as the extension gene, controls production of pigment in melanocytes. Melanocytes are cells that control the coloration of skin or fur. The dominant form of the gene, the "E" allele, allows the dog to produce eumelanin, which is a black pigment. This can appear either as E/e or E/E. A mutation in the MC1R gene causes the pigment-producing cells to only produce phaeomelanin instead of eumelanin. This turns all the eumelanin in the coat to phaeomelanin. This form of the gene is represented as the "e" allele. The "e" allele is recessive (e/e), meaning that a dog must have two copies of the MC1R mutation to express the yellow or red coat color. Recessive red can mask other color variants. It can even mask the merle coloration.
A third allele exists in the extension gene: Em. Em is also dominant. This means a dog can have Em/e or Em/Em in order for the Em coloration to be expressed. This causes the dog to have a black mask on their face. This mask is also known as a melanistic mask. This allele acts similarly to the E allele in that it causes a black-based coat. Because it is dominant, a dog only needs one copy of the Em allele to express this trait. In solid black dogs with a copy of the Em allele, the mask is hidden. This is because the mask and the fur color are both black, and the mask then becomes “invisible.” However, the dog can still pass on the melanistic mask to future offspring, even if the mask cannot be seen.
The e/e genotype can vary in expression between different breeds. In some breeds, the difference between a black/brown dog and a yellow dog is obvious (i.e. Labrador Retrievers). However, in other breeds, such as Cocker Spaniels, this difference may be more subtle. Other breeds express the e/e phenotype as a red color.
It is important to note that the extension gene is only one of four important genes in determining the coat color of a canine. The dog's color can vary greatly with different mutated alleles on other genes. Dogs that are e/e will always be yellow. However, there is a great deal of variation of dogs that are E/E or E/e depending on the B-Locus, A-Locus, K-Locus, and D-Locus. To find out more about these variations, make sure to read the corresponding articles.