K Locus – Black Dog Coat Color
The symbol K was chosen to denote the locus causing black dog coat color which is inherited as a dominant. The identification of dominant black (formally, an allele of the “K locus”) relied on two major advances in dog genetics: the sequencing of the dog genome and recognition that the distinctive genetic structure of dog breeds allows for efficient gene mapping. The K locus is identified as b-defensin 103 (CBD103). Its protein product binds with high affinity to the Mc1r and has a simple and strong effect on pigment type-switching in domestic dogs.
The K locus has three alleles with a simple dominance order: Black (KB) > brindle (kbr) > yellow (ky)
In the vast majority of dog breeds, in order for solid eumelanin coat colour (black, brown and grey) to occur the dog must have at least one E or EM allele at MC1R and at least one dominant allele at the K locus.
A single copy of the kbr allele in the presence of a ky allele is sufficient to cause the dog to express the phenotype known as brindle. Brindle in dogs consists of alternate stripes of phaeomelanin and eumelanin of various shades. In some dogs, there is such a preponderance of eumelanin that the dog appears virtually black whereas in other individuals the eumelanin stripes are very thin. Brindle occurs over the entire body in dogs with an ay allele but only on the ventral surfaces in dogs with an at/at genotype.
Dogs with a ky/ky genotype at this locus could be fawn, wolf sable or eumelanin-and-tan depending on their genotype at ASIP.
Kerns, J.A., Cargill, E.J., Clark, L.A., Candille, S.I., Berryere, T.G., Olivier, M., Lust, G., Todhunter, R.J., Schmutz, S.M., Murphy, K.E., et al. (2007). Linkage and Segregation Analysis of Black and Brindle Coat Color in Domestic Dogs. Genetics 176, 1679–1689.