Dominant black

Dominant black is a gene that is pretty much unique to dogs.The gene encodes for a protein of the family of β-defensins which play a role in the immune system. β-defensins have antimicrobial activity and form an important part of the defenses of an individual against bacteria. Surprisingly this protein also binds to the MC1R receptor and thus has a strong effect on the switching of the pigment type between pheomelanin and eumelanin.

A mutation in this gene is the cause of a black coat in at least 50 breeds. It blocks the effect of the Agouti gene because the β-defensin protein binds stronger to the MC1R receptor than both ASIP and α-melanocortin.

The wild type allele for this gene k^y has no effect on the MC1R receptor.

The mutation K^B is a deletion of three bases which represent a single amino acid (G23). This is dominant to k^y. Some breeders have indications that in certain breeds at least it's incomplete dominant and that in heterozygous dogs the effect of Agouti (ASIP) is still visible.

The third allele is k^Br which is dominant over k^y and recessive to K^B. This allele enables the expression of the Agouti (ASIP) gene but in brindling pattern. Brindling is a pattern with stripes that run from the back to the belly (combined with a lot of randomness).
There is no genetic test available yet because the exact genetic mechanism isn't known yet.

Research of the DNA (McNamara) in the dark and light areas of a brindle coat showed differences in methylation. Methylation is an old mechanism that regulates the expression of genes. In the case of the K-locus this means that in the light areas the CBD103 gene was methylated and in the dark areas it was not. How this is regulated in the case of this gene is not clear yet.