The B locus: black or brown
Brown — also called chocolate — is controlled by the B locus, which governs pigment colour itself. In its homozygous recessive form it has a lightening effect on eumelanin (the black‑based colours) only; it has no effect on phaeomelanin (the red‑based colours).
The Brown locus is believed to code for an enzyme, tyrosinase‑related protein 1 (TYRP1), which catalyses the final step in eumelanin production — converting the last brown intermediate (dihydroxyindole) into black pigment. In other words, all dogs start out brown, and it is this final step that pushes the colour on to black.
How the genotypes read
When brown (b/b) is expressed, it simply means that last step of eumelanin production was never completed — it is not a genetic defect. In an ee red dog that is also b/b, the dog will be red or yellow but carry brown pigment on the nose, eye rims and lips, which is the giveaway to its true colour.
What chocolate looks like
The pigment granules produced by b/b are smaller, rounder and lighter than those of a black dog, and the iris of the eye is lightened too. Chocolate therefore comes in many shades, from a dark bitter‑chocolate to a warm milk‑chocolate, often depending on coat stage, season and the light.
