Agouti

The agouti signaling protein (ASIP) gene, or Agouti, interacts with the melanocortin 1 receptor (MC1R) to regulate the switch from eumelanin (present as black, brown, cinnamon, or their respective dilutions) to phaeomelanin (present as red or yellow or their respective dilutions) pigment deposition in the hair shaft at certain points in the growth cycle of the hair. This results in individual banded hairs with the visual effect of an intermediate color between the two pigments. The length of the respective bands modulates the visual effect as well.

In domestic cats, a 2 bp deletion (c.123delCA) in the agouti signaling protein disrupts protein function. This mutation has an autosomal recessive mode of inheritance, thus when two copies are present, pigment switching is prevented from occurring. This results in a hair of uniform color (often called solid or “self-colored”). A separate gene causes patterns of dark stripes or blotches (tabby patterns) to be interspersed with lighter hairs. Hairs in the darker stripes do not undergo the shift between black and red-yellow pigment production during their growth, and thus remain uniformly dark. The areas outside of the dark tabby stripes are composed of agouti-banded hairs. In a non-agouti cat (a/a), the tabby pattern is still present, but it is just not visible since all the hairs in the pattern and outside the pattern are a uniform color.

There are instances where the tabby pattern can be seen despite a homozygous recessive (a/a) genotype at the agouti locus. A sex-linked red/orange cat will exhibit the tabby pattern regardless of whether it is agouti (A/A or A/a) or non-agouti (a/a). Under certain lighting conditions, faint tabby patterns are occasionally observed on the body in a/a cats, especially in kittens with the dilution variant. Lastly, melanistic (a/a) Bengal cats will often exhibit a faint tabby pattern as well.