The B, or brown locus (known as TYRP1, on chromosome 8), is well known in dog breeds (like the chocolate Labrador colour) and is also seen in Dexter cattle (a rich reddish brown colour). It can only affect eumelanin (black pigment) and in fact the brown colour comes from modification of this black pigment and not red pigment.

The relevance of this locus in Highland cattle coat colouring is unknown, although I have wondered about the mahogany gene that we have in our herd being at the locus. Or perhaps this colour is more like the mahogany colour of Ayrshire cattle, which appears to be a variation in shades of phaeomelanin that originate from the E locus. Interestingly this breed emanates from Scotland pre-1800's and is the only other breed that I have heard of that has the gene for crop ear.