<P> The social significance of differences in skin color has varied across cultures and over time, as demonstrated with regard to social status and discrimination . </P> <P> Melanin is produced by cells called melanocytes in a process called melanogenesis . Melanin is made within small membrane--bound packages called melanosomes . As they become full of melanin, they move into the slender arms of melanocytes, from where they are transferred to the keratinocytes . Under normal conditions, melanosomes cover the upper part of the keratinocytes and protect them from genetic damage . One melanocyte supplies melanin to thirty - six keratinocytes according to signals from the keratinocytes . They also regulate melanin production and replication of melanocytes . People have different skin colors mainly because their melanocytes produce different amount and kinds of melanin . </P> <P> The genetic mechanism behind human skin color is mainly regulated by the enzyme tyrosinase, which creates the color of the skin, eyes, and hair shades . Differences in skin color are also attributed to differences in size and distribution of melanosomes in the skin . Melanocytes produce two types of melanin . The most common form of biological melanin is eumelanin, a brown - black polymer of dihydroxyindole carboxylic acids, and their reduced forms . Most are derived from the amino acid tyrosine . Eumelanin is found in hair, areola, and skin, and the hair colors gray, black, blond, and brown . In humans, it is more abundant in people with dark skin . Pheomelanin, a pink to red hue is found in particularly large quantities in red hair, the lips, nipples, glans of the penis, and vagina . </P> <P> Both the amount and type of melanin produced is controlled by a number of genes that operate under incomplete dominance . One copy of each of the various genes is inherited from each parent . Each gene can come in several alleles, resulting in the great variety of human skin tones . Melanin controls the amount of ultraviolet (UV) radiation from the sun that penetrates the skin by absorption . While UV radiation can assist in the production of vitamin D, excessive exposure to UV can damage health . </P>

Which pigment determines the colour of human hair
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