Genetic researchers have identified a certain mutation in humans that causes defective tyrosinase that is temperature sensitive in the same way as in Siamese cats. The resulting genetic disorder is called type 1b temperature-sensitive oculocutaneous albinism (OCA 1b TS) (Giebel et al. 1991). The mutation causing OCA 1b TS has been identified as a different missense SNP in TYR, but it has similar effects on phenotype.
Left: The first champion Siamese cat in the United Kingdom (1898), "Wankee." Little is known about the history of the Siamese cat before the 19th century.
Unfortunately, temperature-sensitive TYR mutations do not affect only hair color (in humans) or fur color (in cats). Melanin is found in the retina and is necessary for normal vision. Defective tyrosinase can lead to incurable vision problems in sufferers of OCA 1b TS and other forms of albinism, including strabismus (crossed eyes), nystagmus (eye "trembling"). Siamese cats are also known to have vision abnormalities. In the past, many had crossed eyes (strabismus), a compensation for the abnormal visual field (Berman & Cynader 1972). In breeding this trait out of Siamese cats, breeders may actually have created cats with worse vision than their ancestors.
Right: Siamese cat with crossed eyes (strabismus). Selective breeding has made this trait rare, though it may actually improve the cats' vision. Image by Mark Ekimov (public domain).