Siamese cats are renowned for their peculiar coat pattern. A cream or white underbelly shades gradually to a medium color on the back, while the ears, face, legs, and tail are very dark. This pattern is called "pointed" or "colorpoints," and it affects not only the coat, but also the eyes, which are always blue.
Pointed cats are partial albinos. Albinism is a mutation in the TYR gene, responsible for the enzyme tyrosinase. Tyrosinase, in turn, is necessary for the production of melanin, or dark pigmentation. Albinism is not a black or white trait; it has shades of gray—literally. In cats, various mutated alleles in TYR can result in phenotypes with different degrees of pigmentation. Each of these phenotypes falls in a specific place in an allelic series with a hierarchical inheritance pattern from dominant to recessive—in other words, some of these traits are dominant over others. The Siamese phenotype falls in the middle of the series, recessive to normal coloration but dominant over a completely white coat.
Above right: Siamese cat with prey. Photo by "emdot." (CC) Some rights reserved.
The albino mutation in Siamese cats results in a defective form of tyrosinase which does not function at normal body temperature. Therefore, dark coloration can only appear in parts of the body that are cooler than the core body temperature. The extremities are always the coolest parts of the body. The face is also cooler because of air passing through the sinuses. The back is warmer than the extremities, being closer to the body core, but it is also exposed. The result is a medium degree of tyrosinase function, resulting in a medium degree of shading
Left: The Siamese colorpoint mutation can be combined with other coat color mutations. This cat has both colorpoints and leucism (white patches). Image by Feliciano Guimarães. (CC) Some rights reserved.
The mutation that causes colorpoints in cats has been identified as a point mutation, or single-nucleotide polymorphism (SNP). It changes glycine to arginine in the tyrosinase protein, and is therefore a missense mutation. The identical mutation is seen in other breeds of cats, including mixed-breed cats that have a colorpoint pattern, so all pointed cats are thought to share a common descent. The original mutation occurred centuries ago in Thailand (formerly Siam), but the details of the history of the Siamese cat are cloaked in mystery. (Lyons et al. 2005)
Right: Two Siamese cats. The colorpoint gene can be found in cats of any base coat color. Image by Loïc Dreux. (CC) Some rights reserved.