Cat Coat Color – A locus
The interaction between two genes, Agouti on A locus and Melanocortin-1 receptor (Mc1r), produces diverse pigment patterns in mammals by regulating the type, amount, and distribution pattern of the two pigment types found in mammalian hair: eumelanin (brown/black) and pheomelanin (yellow/red). Agouti encodes for agouti signalling protein. The dominant is the wild-type A and it causes hairs to be black pigmented at the tips and orange pigmented at the roots. The recessive non-agouti or “hypermelanistic” allele, a, prevents this shift in the pigmentation pathway. When the cat is homozygous (genotype aa), it will result in black pigment production throughout the growth cycle of the hair. The aa genotype will hide the tabby pattern. The A to a mutation on A locus does not have a discernible effect on red or cream colored cats. This results that these cats display tabby striping independent of their genotype at this locus. This explains why you can usually see the tabby pattern in the orange patches of non-agouti tortoiseshell cats, but not in the black or brown patches.
Kerns, J.A., Newton, J., Berryere, T.G., Rubin, E.M., Cheng, J.-F., Schmutz, S.M., and Barsh, G.S. (2004). Characterization of the dog Agouti gene and a nonagoutimutation in German Shepherd Dogs. Mamm Genome 15, 798–808.