Coat colour B locus / brown, chocolate and liver (TYRP1-related) - Dog

What does this test examine?

This DNA panel examines variants in the TYRP1 gene, better known as the B locus or Brown locus. The panel is designed to detect recessive B-locus alleles that can change black eumelanin into brown, chocolate or liver. It includes the classic B-locus variants and additional variants relevant in breeds such as Lancashire Heeler, Siberian Husky and Australian Shepherd.

What does the B locus mean for the coat?

TYRP1 affects the quality of dark eumelanin pigment. When a dog carries two recessive b alleles, black pigment becomes brown. This can be visible in coat, nose, lips, eyelids, foot pads and nails. In red or yellow dogs the coat effect can be less visible, but nose and foot pads may still show brown pigment.

Colour names differ by breed and context. The same genetic mechanism may be called brown, chocolate, liver, red in Australian Shepherds or part of combinations such as lilac or isabella when dilution is also present.

Inheritance and result

The B locus is autosomal recessive. A dog with at least one functional B allele keeps normal black eumelanin. A dog with two recessive TYRP1 alleles, even when they are different B-locus variants, produces brown eumelanin. This is why interpretation of the full panel is more important than any single variant alone.

• B/B: no tested brown variant detected.
• B/b: carrier of a brown variant; usually not brown from this variant alone.
• b/b: genetically brown/chocolate/liver when other colour genes allow dark pigment expression.

Practical value of this test

This test is especially useful for breeders planning or avoiding brown, chocolate, liver, lilac or isabella combinations. Because recessive B-locus variants can remain hidden for generations, the panel helps identify carriers early and choose matings deliberately.

• Helps explain why black-pigmented areas become brown.
• Identifies carriers before breeding decisions are made.
• Supports planning of brown, chocolate, liver, lilac and isabella with E, K, A and D locus results.
• Reduces surprises in litters and improves communication with buyers.
• Useful in breeds with both classic and breed-associated TYRP1 variants.