In cats, a brown coat colour can be caused by several mutations to the gene for tyrosinase-related protein 1 (TRYP1). This gene is also known as the Brown Locus, or B-Locus. The phenotypes resulting from the recessive B-Locus mutations cause hair that would otherwise present as black to develop as brown instead.
There are two different sub-varieties of brown in cats: Cinnamon, which is a lighter brown also sometimes designated as red; and Chocolate, a darker brown which may also be called chestnut.
Brown colouration resulting from the B-Locus is only visible if the cat in question can produce dark pigment in the first place. For example, cats affected by an Extension Locus (E-locus) mutation or complete albinism will not display any brown colouration.
Inheritance
There are two variant mutations: b, associated with Chocolate, and b^l, associated with Cinnamon. The b mutation is itself dominant over b^l. Both mutations are recessive to the wild-type, B. Therefore, a cat with the genotype b/b^l will have a Chocolate phenotype, and a cat with the genotype B/b or B/b^l will have a normal, non-brown phenotype.
Relevant tests
• K755
• K756
