After reading ‘Cat Sense’ by John Bradshaw, I wanted to find out more about interactions between genes and the environment. Bradshaw mentioned that the darker patches of fur on the face, paws, ears and tail of Siamese cats are caused by a temperature sensitive mutation which causes the hairs to be darker the colder the temperature. This means that when Siamese kittens are first born they are white all over because their mother’s womb is uniformly warm. However, as they grow, the coolest parts of their bodies – the extremities including face, paws, ears and tail – become darker in colour. Furthermore, cats living near the equator or in very warm houses tend to be lighter than those living in cooler climates.
The colour of a Siamese cat has been compared to oculocutaneous albinism in humans which causes white skin, very pale hair and light-coloured irises. In humans, oculocutaneous albinism is the result of a genetic change producing a
temperature-sensitive tyrosinase polypeptide. This protein’s enzymatic activity is reduced even at 31°C and is almost abolished at 37°C. Tyrosinase is the enzyme that catalyzes the production of melanin. Therefore, the higher the temperature, the less enzyme activity and the less melanin produced, resulting in albinism. It is thought that a very similar phenomenon is happening in cats, where enzymes are directly affected by their surrounding temperature are controlling the fur colour.