Brown or chocolate coat color in cats is due to a mutation in the TYRP1 gene, which affects the production of black eumelanin, resulting in a chocolate brown shade.
Definition
Chocolate coat color in cats results from alterations in the TYRP1 (tyrosinase-related protein-1) gene or B ("brown") locus, which affects the production of eumelanin, the dark pigment responsible for black or brown color. TYRP1 encodes an enzyme involved in the final stages of melanogenesis, where tyrosine is converted to eumelanin.
Genetic basis
The variant responsible for the chocolate color in cats is part of a B > b > bl allelic series. The B allele corresponds to the wild-type phenotype that produces a black coloration, whereas the b allele results in a chocolate brown shade. Finally, the bl allele results in the cinnamon phenotype. Two copies of the b allele, specifically the c.1261+5G>A T variant, are required to express the chocolate-colored coat. The presence of a single copy of this variant can produce a chocolate phenotype if it is in compound heterozygosis with a copy of the bl (cinnamon) allele. In the absence of any copy of the bl allele, a single copy of the b allele will not express the chocolate phenotype, but will be transmitted to the offspring of the feline with a 50% probability.
Other relevant information
An interesting fact about cats with brown coats is that the mutation in the TYRP1 gene that causes this coloration may also influence their eye health. Because of the lower amount of eumelanin in the iris, brown cats may have lighter-toned eyes and, in some cases, be more sensitive to intense light.
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