The B locus determines what colour the dark pigment will be. Black or Brown. Black [ B ] is the default. A gene mutation causes the pigment to be made brown [ b ] instead of black. This affects the coat colour as well as eye colour and leather. The term "leather" includes nose, eye rims, lips and pads. Order of dominance is BB > Bb > bb. In Havanese, the brown dog is called chocolate. Brown [ bb ] cannot have any black pigment anywhere including eyes and nose. Brown can appear as the dark coat in any mixed dark/light coat, so there may be Chocolate Brindles, Chocolate Sables and Chocolate & Tans. If there is a chance meeting of two recessives [ bb ] and [ ee ], clear dogs with liver pigment may result. Remember. Phenotype is what you see and genotype is the possible genetic codes behind it.
The dog carries either one copy or two copies of the dominant B allele. The dog will have a black-based coat. If he carries both the dominant and recessive copies of the B allele, then he will have black-based coat, but carry (hidden) the allele for the brown coat.
Phenotype - dog displays a black based coat
Genotype - [ BB ] or [ Bb ]
[bb] BROWN: Brown is recessive to black and can only be expressed where there are two copies of the recessive gene (bb). A brown dog does not carry the capacity to make black at all. The brown gene affects the colour of the leather and eyes as well. The nose and pads will be brown and the eyes may be lighter coloured.
Phenotype - dog displays brown
Genotype - [ bb ]