Their numbers in any one region of the body, which range from about 1,000 to more than 2,000 per square millimetre, are roughly the same within and between races; the blondest whites have as many as the darkest blacks. Color differences are due solely to the amount of melanin produced and the nature of the pigment granules. When the skin becomes tanned on exposure to sunlight, the melanocytes do not increase in number, only in activity." ("Integumentary Systems, Pigmentation",CD. Version 97. Encyclopaedia Britannica, Inc., 1997.)
Melanocytes are also responsible for the form of cancer called melanoma. Melanoma is caused by ultraviolet û @UV û radiation damage to melanocytes. Repeated exposure to UV can cause cancerous mutations.
In non-Caucasian races, melanin production is continuous, so the skin is always pigmented to some degree. In these races the incidence of skin cancer is much lower because cells are constantly protected from UV radiation by melanin.
Melanocytes actually produce two different pigments: eumelanin (brown) and phaeomelanin (yellow and red). Redheads happen to produce more phaeomelanin and less eumelanin, which is why they don't tan very well. In albinos, the chemical pathway that produces melanin does not function because an enzyme called tyrosinase is missing. Therefore albinos have no melanin in their skin, hair, or irises.
