The 'K' Locus
Alleles at this locus can only lead to the production of eumelanin (black pigment). The Kbr allele accounts for the brindle coat colour pattern seen in dogs, and was until recently thought to account for Highland cattle as well. As mentioned on the A locus page, brindle appears to be determined at the agouti locus (Abr) in Highland cattle.
This locus also has an allele for dominant black (K) as well as the wild type allele (k+) in other species but has not been proven to exist in cattle.