Genetics
The K Locus: Dominant Black, Brindle, and the Color Switch in Dogs
The K locus is called the 'color switch' because it controls whether A locus patterns (sable, tan points, agouti) are expressed at all. Here's how it works.
March 1, 2024 · 5 min read
What the K Locus Controls
The K (Black) locus controls the ASIP gene pathway that determines whether agouti signaling proteins can influence coat color. There are three K locus alleles: KB (dominant black, which masks all A locus patterns), kbr (brindle), and ky (allows A locus expression). Dominance hierarchy: KB > kbr > ky.
The Three K Locus Alleles
- KB (Dominant Black): One or two copies produce solid eumelanin expression. The A locus cannot be seen — a dog could carry phantom or tan point genes that are completely hidden. Most solid-colored dogs in many breeds carry KB.
- kbr (Brindle): Produces a striped eumelanin over phaeomelanin pattern. The brindle pattern requires at least one kbr allele and no KB allele. kbr/kbr or kbr/ky produces brindle.
- ky (Allows agouti expression): Both copies must be ky/ky for A locus patterns (sable, tan points, solid recessive black, wolf agouti) to be visible.
Why Solid Black Dogs Can Be 'Phantom Carriers'
A dog with KB/ky at the K locus and at/at at the A locus appears completely solid black. But breed it with a ky/ky dog, and suddenly phantom (tan-pointed) puppies can appear that seem to come from nowhere. This is one of the most common 'surprise color' situations in breeding programs — a solid black dog producing phantom pups because the KB allele was masking the at/at genotype.