K Locus: Dominant Black and Brindle Genetics in Dogs

The K locus is one of the most important yet least understood color loci in dog genetics. It determines whether the A locus — which controls sable, agouti, tan points, and recessive black — even gets expressed at all. If a dog is dominant black at K, no A locus color will show, regardless of what A alleles the dog carries.

The Three K Alleles

KB (dominant black): A dog with at least one KB allele will be solid black — or whatever color the B and D loci produce. If the dog is bb (chocolate) and carries KB, it will be solid chocolate. If it is dd (dilute) and KB, it will be blue. KB completely masks the A locus.

kbr (brindle): Brindle is the striped pattern seen in Boxers, Mastiffs, French Bulldogs, and many other breeds. The kbr allele is dominant over ky but recessive to KB. A dog must be kbr/ky or kbr/kbr to express brindle, and it must also be ky at one or both positions to allow A locus expression.

ky (non-solid / A locus expressed): Dogs that are ky/ky allow the A locus to determine coat color. This is why a ky/ky dog might be sable, agouti, tan-pointed, or recessive black depending on its A locus genotype.

How K Interacts With Other Loci

The K locus sits on top of the A locus in the dominance hierarchy. A dog can carry beautiful tan-point genetics (at/at) at the A locus but show none of it if it carries even one KB allele.

Example: A French Bulldog that is KB/ky will appear solid (fawn, chocolate, blue, or black depending on B and D) even if it is at/at at the A locus. Cross it with another KB/ky dog and 25 percent of puppies will be ky/ky — and those puppies will express whatever A locus alleles they carry.

Breeds Where K Locus Matters Most

• French Bulldogs: Brindle (kbr) is common; solid fawns are ky/ky
• Boxers: Virtually all colour patterns are controlled by K and A loci
• Mastiffs: Brindle vs fawn is determined by K locus
• Labrador Retrievers: Almost all Labs are KB/KB or KB/ky, which is why tan-pointed Labs are extremely rare
• German Shepherds: Most are ky/ky, allowing full A locus expression
• Australian Shepherds: ky/ky, which is why merle, tri, and sable patterns all express

Testing for K Locus

Embark, PawPrint Genetics, and Animal Genetics all test for K locus. If you are breeding for specific patterns — especially brindle or tan points — knowing your breeding pair's K locus genotype before pairing is essential for predicting puppy colours accurately.