chocolate and cinnamon

 
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The B locus (B stands for brown, or possibly black) is responsible for the colors 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 certain breeds of cat.

 Chocolate is also found in dogs, but cinnamon is unique to cats.  From Mr.OutdoorGuy on Flickr.

Chocolate is also found in dogs, but cinnamon is unique to cats. From Mr.OutdoorGuy on Flickr.

Genotype Color
BB Black
Bb Black (carries chocolate)
Bb1 Black (carries cinnamon)
bb Chocolate
bb1 Chocolate (carries cinnamon)
b1b1 Cinnamon

Remember that these colors combine with sex-linked red. Red overrides all of the black-based colors by inhibiting production of eumelanin, but tortoiseshell can combine with any of these colors to make the standard black tortoiseshell, chocolate tortoiseshell, or cinnamon tortoiseshell.

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