What Causes The Siamese Colouration In Cats?

The dark points of a Siamese cats is due to a genetic freak, a form of albinism.
Albino animals are unable to synthesize melanin, the pigment that creates colouring of hair, skin, and eyes. Siamese colouration is due to the "Himalayan" gene, which has the same location of the albino gene.
Himalayas animals are only able to synthesize melanin at places of low body temperatures: tail, paws, ears, nose, and legs. Hence, only these places are dark-coloured.
The blue eyes of a Siamese cat are also due to low distribution of melanin in the iris. Siamese cats' eyes reflect back red, rather than green, because of a colourless retina.
The Himalayas gene is also associated with vision problems. Hence, many Siamese cats are cross-eyed.
In addition to the classic dark brown "points," Siamese may have purplish (lilac), grey, or striped (lynx) body tips.