Allelic series at the Colour locus in domestic cats

    Like other mammals, some cats are albino due to defects in tyrosinase metabolism. The illustration shows an albino (a) [left] and her colour-point (chocolate lynx-point non-albino) sibling (b) [right]. The litter of kittens in the lower photo includes an albino and a colourpoint kitten (third from the left and right, respectively). The kittens have not yet developed colour points.

    Cats and other mammals express two forms of melanin, called eumelanin (black) and phaeomelanin (orange), respectively. Mutations at the tyrosinase locus (TYR) affect both pigments, and produce dilute forms of each. The typical "points" of siamese and burmese cats result from a temperature-sensitive form of albinism, in which eumelanin pigment develops only in the cooler extremities, such as ears, paws, and tail. The degree of darkness of these features is determined by the exact form of eumelanin, which is determined by a series of four alleles:

C [or c+] (full colour) > cb (burmese) ~ cs (siamese) > c (albino).
C (c+) is fully dominant to those below it, cb has brown points, and cs black points. Show-quality burmese and siamese cats are homozygous cbcb and  cscs respectively. Heterozygotes (cb/cs) show a colour called tonkinese that is intermediate between burmese and siamese colouration. The first three alleles are fully dominant to c.

    For more on cat coat colour genetics see [].

Photo from Imes et al. 2005 Animal Genetics 37, 1315; additional text material ©2011 by Steven M. Carr