Little postulated the allele A^S causes solid or self-colored dogs. Later authors, such as Willis (1989) and Sponenberg (2001) have for some reason begun to call this A^S allele "dominant black". Kerns et al. (2003, 2005) have recently shown that this "dominant black" is not caused by an agouti allele using DNA studies. "Dominant black" by their definition is a genotype that is epistatic to fawn, sable, etc. agouti phenotypes which occurs at another locus, K (for blacK). Several dog breeders have suspected this for some time. The “K locus” is located on an entirely different chromosome than the agouti locus. The Stanford group has decided 
to refer to the three alleles as follows :

·        K^B  = solid black (eumelanin pigmentation only)

·        k^br  = brindle 

·        k^y   = expression of agouti alleles.

The K^B allele

The K^B allele is actually the allele causing this type of solid black, "solid" does not mean that no white markings occur. The gene is a beta-defensin. This family of genes has been known to play an important role in immunity in humans, mice and other animals previously. This is the first time that a defensin gene has been shown to affect pigmentation though. This was announced by Sciencexpress on 18 October 2007. The full article title is : “ A ß-Defensin Mutation Causes Black Coat Color in Domestic Dogs”.

The brindle allele k^br

The Brindle allele is part of the K series and as such is on a different chromosome than is MC1R (the E locus). The presence of brindle is dominant to fawn so only one copy of k^br is required. Likewise since k^br is recessive to K^B, no K^B allele can be present in the dog. Therefore all brindle dogs are either k^br/k^br or k^br/k^y.

k^y/k^y  Phenotypes

Dogs which have two recessive alleles k^y/k^y can express a variety of phenotypes. All fawn or sable dogs are k^y/k^y, whether they have a melanistic mask or not. The Stanford group chose to name this allele k^y since it allows yellow or phaeomelanin pigment to show. Where it shows depends on the alleles at the agouti locus.

References :  www.sciencexpress.org / 18 october 2007
                      http://homepage.usask.ca/~schmutz/dogcolors.html

JMV – December 2007