The K locus plays a pivotal role in coat colour. This locus is a relative newcomer in our understanding of canine colour, and includes traits formerly attributed by some to other genes.

The dominant allele in the series is K^B, which is responsible for self-colouring, or solid coloured fur in pigmented areas. This trait was formerly attributed to the Agouti (A) locus as A^S, but recent breeding studies had shown this not to be the case.

There are two other alleles, k^br, and k^y. K^B is dominant to both k^br and k^y, while k^br is dominant only to k^y. k^br is responsible for the brindle. The recessive allele, k^y, allows the basic patterns of the A locus to be expressed. So too does the k^br allele, but with brindling of any tan, fawn, or tawny areas. Any animal with at least one K^B allele will be self-coloured.

Any animal with at least one k^br allele, and no K^B alleles will be brindled on agouti background (see A locus). Any animal with two k^y alleles will show agouti patterns (see A locus). and k^y

LABOKLIN can presently test for these two alleles. In some breeds, where no brindle is present, this represents a complete analysis of the locus. An example would be the Pug. In breeds where the breed standard disqualifies all but self-colored dogs, testing for these two alleles is once again all that is needed. Any animal with two K^B alleles cannot produce anything except self-coloured offspring. A prime example here is the Labrador retriever. In breeds where many variations are allowed, these tests can help predict the probability of potential litters to include fawn, sable, tawny, tan point, tricolor or recessive black puppies.