The B Locus
The B locus, also called the Brown Locus, is responsible for turning the eumelanin pigment (which is normally black) to brown. The wildtype of this Locus is dominant and is represented by “B”. Any pigments affected by eumelanin are black. The recessive mutation of this locus is represented by “b”. Any pigments affected by eumelanin are brown. When a rat has no other mutations on the other loci (is wildtype on all other loci) and is recessive on this locus, the colour is known as Chocolate Agouti. When it is recessive on the A locus as well as on this locus, the colour is known as Chocolate.
Alleles on the B locus are as follows:
BB and Bb animals will have black eumelanin, while bb animals will have brown eumelanin.
Chocolate Unmarked Harley Standard Ear
IMGN Brownie & IMGN Gingerbread
Agouti (Bottom) vs Gingerbread (Top)