mike's web log

Among domestic cats, various color mutations appears in a considerable percentage of the population. The most important color variations controlled by single genes are the blotched tabby (the mc, or t^b, gene), in which the tabby stripes are intensified and often fuse together to form black whorls or blotches; Abyssinian tabby (T^a), in which tabby stripes are reduced to a vestigial ticking on the legs; non-agouti (a), in which the pigmentation of the hairs is uniform along its length, resulting in a solid-colored cat, typically all black; dilute, in which the color pigments are scattered in in clumps in the hairs, resulting in a smoky blue or fawn or cream color; or orange (O). The mutant O gene has the peculiarity of being carried on the X chromosome, and because females have two X chromosomes while males have only one, this so-called sex-linked trait produces a sexual divergence in color possibilities. A male with an O gene is just orange. A female, however, can carry an O gene on one of her X chromosomes but not on the other, resulting in the expression of both orange and non-orange coat colors simultaneously; this gives rise to the combined orange and black patterns of the (almost always female) calico- or tortoiseshell-colored cats.

A pure white coat in cats also has the odd characteristic that is common to many other mammals. The cells that produce colored skin and hair pigments derive in fetal development from the cells in the neural crest, an embryonic structure that also generates the brain and spinal cord. As a result, you usually can't lose your color without losing your brain as well, or at least parts thereof. Cats that carry the W gene, preventing the development of color pigment cells, also often suffer from neural defects such as deafness or blindness.