The genetics of the Siamese coat color is a form of Himalayan albinism which is found in many animals including the rabbit (Himalayan rabbit) and mouse. In the Siamese cat more pigmentation is displayed than normal. The eyes are blue rather than pink and the body is slightly shaded rather than stark white.
Apart from these differences, the Siamese cat’s appearance (phenotype) is typical of Himalayan albinism and the kittens are born devoid of pigmentation. Kittens begin to form pigment on their noses and ears within a few days.
Most people know that the production of pigment in the hair of the Siamese cat is temperature dependent. Pigment forms on the points (the extremities) of the cat because the temperature of these areas is sufficiently low to affect the gene. Apparently some cats respond to temperature changes while others do not. The cooler the ambient and body temperature the darker the coat color.
The Siamese cat albinism allele affects the cat’s eyesight because the visual pathway is disrupted. The cats are prevented from having full binocular vision although they compensate neurologically for the abnormality. It causes the well-known cross eyes or squint of the Siamese cat.
The Siamese cat is a non-agouti black with the addition of the Siamese allele. The genetics for the seal point are written: aaB-cscsD-. The color of the fur of the extremities of the cat is not black but a dark seal brown. Robinson’s Genetics describes it as a “degrading of color intensity” which is characteristic of this albinism allele.
I hope this helps.
Source: Robinson’s Genetics for Cat Breeders and Veterinarians.