The B locus is responsible for chocolate and cinnamon. These colors arise from modifications of eumelanin, the black pigment, so they are referred to as black-based.
The B locus has three alleles. B is associated with black pigment, and is the most dominant allele. Chocolate color, associated with the b allele, is caused by production of a brown pigment instead of black. Cinnamon is a lighter brown or reddish-brown, and is associated with the b1 allele, which is recessive to both the B and b alleles.
These colors are caused by a modification in the pathway that creates eumelanin, and therefore do not significantly affect phaeomelanin. Because the b and b1 alleles can affect spacing of melanocytes as well as the structure of eumelanin, red bb cats may appear somewhat lighter than red B- cats, but since other factors affect the color of a red cat, this is not a reliable way to tell the genotype of a red cat at the B locus.
Chocolate and cinnamon are both quite rare colors, cinnamon in particular, and are usually only found in purebred cats. Cinnamon and its derivatives are not even recogni
|Bb||Black (carries chocolate)|
|Bb1||Black (carries cinnamon)|
|bb1||Chocolate (carries cinnamon)|