K Locus · CBD103 (β-defensin) & brindle modifier

K Locus: Dominant Black

The gatekeeper for the A locus — decides whether a dog shows solid color, brindle stripes, or an agouti pattern like sable or tan points.

Alleles

KB: Dominant black: Solid eumelanin coat regardless of A locus genotype. A single copy completely masks sable, tan points, and all other agouti patterns. Most dominant allele at this locus. The color of the solid coat (black, chocolate, blue, lilac) is set by B and D loci.
kbr: Brindle: Creates tiger-stripe patterning of eumelanin over a red/phaeomelanin base. Only expresses in the absence of KB (one or two copies). Extremely common in French Bulldogs. Brindle intensity ranges from reverse brindle (mostly dark) to light brindle (a few stripes on a fawn base).
ky: Non-solid (recessive): Allows the A locus pattern (Ay sable, aw agouti, at tan points) to fully express. Only ky/ky dogs show A locus patterns. Most recessive allele here. Does not produce a visible effect by itself — ky/ky simply 'gets out of the way' and lets A locus work.

What the K locus does

KB (dominant black, gene CBD103/β-defensin 103) floods the coat with solid eumelanin by constitutively activating the MC1R receptor — essentially overriding the A locus signal. One copy of KB is enough to produce a solid black coat, regardless of what the Agouti locus says.

The dominance order is KB > kbr > ky. A dog needs only one KB allele for solid color; two kbr alleles (or kbr/ky) for brindle; and specifically ky/ky to let the A locus patterns show.

Brindle (kbr) — how stripes form

Brindle creates irregular vertical eumelanin stripes over a phaeomelanin (red/fawn) base. The mechanism is not fully elucidated but relates to kbr's intermediate signal strength — stronger than ky's silence but weaker than KB's full activation. The result is a patterned output: some areas of the coat produce eumelanin (the stripes) while the base remains phaeomelanin.

Because KB is dominant over kbr, a solid black dog can carry brindle hidden underneath. KB/kbr dogs are phenotypically solid — they look no different from KB/KB dogs — but they can produce brindle puppies when paired with a dog that provides ky or kbr.

Brindle intensity varies widely: a 'reverse brindle' dog (often called near-black brindle) is heavily striped and looks nearly solid; a 'light brindle' on a fawn base has just a few stripes. Modifier genes and the dog's specific B and D locus genotype also affect the exact appearance.

How K interacts with E and A

K sits in the middle of the epistasis hierarchy. K sits below E: a e/e recessive red dog shows red regardless of what K says, because there is no eumelanin to make solid or brindle with.

K sits above A: KB and kbr suppress A locus patterns. Only ky/ky dogs express sable, tan points, or wild agouti. A solid black French Bulldog (KB/ky, at/at) may carry hidden tan-point genetics that appear in ky/ky puppies.

K locus in French Bulldogs specifically

The K locus is one of the most commercially significant loci in French Bulldogs. Brindle (kbr) is extremely common and highly prized. Many Frenchies that appear solid black are actually KB/kbr — carrying hidden brindle. Fawn Frenchies are typically ky/ky with Ay sable at the A locus, allowing the warm fawn color (often with Em mask) to show.

A French Bulldog breeder who understands K locus genotypes can predict with confidence which pairings will produce brindle, solid, or fawn puppies — instead of being surprised each litter.

K Locus FAQ

How is brindle inherited in French Bulldogs?

Brindle comes from the kbr allele at the K locus. Because dominant black (KB) outranks brindle, a solid dog can carry brindle hidden. Two kbr-capable parents (neither having two KB copies) can produce brindle puppies. A kbr/ky dog is visibly brindle; a kbr/kbr dog is also brindle (and breeds true for brindle when paired with any non-KB dog). KB/kbr dogs look solid but carry brindle.

What does ky/ky mean in dog genetics?

ky/ky means the dog has two copies of the recessive non-solid allele at the K locus — no dominant black and no brindle. This means the K locus 'gets out of the way,' allowing the A locus (Agouti) pattern to fully show. A ky/ky dog will be sable, tan-point, agouti, or recessive black depending on what its A locus genotype says — provided it is also not e/e at the E locus.

Can a solid black dog produce brindle puppies?

Yes, if the solid black dog is KB/kbr (one dominant black, one brindle allele). KB/kbr dogs look completely solid — you cannot tell them from KB/KB by appearance. When bred to a dog that provides ky or another kbr, roughly 50% of puppies can be kbr/ky or kbr/kbr and therefore express brindle. DNA testing is the only way to know if a solid dog carries hidden brindle.

What is the difference between dominant black and recessive black?

Dominant black (KB) requires only one copy and comes from the K locus (CBD103). It overrides the A locus entirely. Recessive black (a) comes from the A locus (ASIP) and requires two copies (a/a) — it only shows in ky/ky dogs where K is not overriding. Both look solid black in the coat, but they behave completely differently in breeding: KB/ky dogs produce both solid and A-locus-patterned puppies; a/a ky/ky dogs paired together produce only recessive black puppies.

What is a 'ghost' or hidden brindle dog?

A 'ghost brindle' or hidden brindle is informal terminology for a dog that carries the kbr allele but does not express brindle visually — usually because it is also KB/kbr (dominant black hides the brindle). The dog looks solid but can produce brindle offspring. DNA testing reveals ghost brindle status with certainty.

Can two fawn French Bulldogs produce a brindle puppy?

Only if both parents carry the kbr allele alongside ky. Two fawn Frenchies are typically ky/ky (allowing fawn to show), but their K locus genotype could be ky/ky (no brindle possible) or kbr/ky (carrier). Two kbr/ky parents have a 25% chance of producing a kbr/kbr puppy — brindle. Two ky/ky parents can only produce ky/ky offspring — no brindle is possible. DNA testing the parents resolves this completely.

Does the K locus affect nose or eye color?

No. The K locus only affects coat pigment distribution. Nose, paw pad, and eye rim color are determined by the B locus (black vs. chocolate) and D locus (full vs. dilute), not by K.

Can a brindle dog have a recessive red puppy?

Yes, if the brindle parent also carries the e allele at the E locus. A kbr/ky dog that is also E/e can produce e/e puppies when paired with another E/e dog — and e/e puppies appear red or cream regardless of their K locus. An e/e kbr/ky dog looks red/cream and hides the brindle entirely.