Human skin color
Human skin color can range from almost black to pinkish white in different people. Skin color has sometimes been used in an attempt to define races. In general, people with ancestors in sunny regions have darker skin than people with recent ancestors in regions that lack much sun light.
Skin color is determined by the amount and type of the pigment melanin in the skin. Melanin comes in two types: phenomelanin (red to yellow) and eumelanin (dark brown to black). Both amount and type are determined by 4-6 genes which operate under incomplete dominance. One copy of each of those genes is inherited from the father and one from the mother. Each gene comes in several alleles, resulting in a variety of different skin colors.
Dark skin protects against skin cancer, mutations in the skin induced by ultraviolet light. Light skinned persons have about a tenfold risk of dying from skin cancer under equal sun conditions. The advantage of light skin is that it lets more sunlight through, which leads to increased vitamin D3 production necessary for calcium absorption and bone growth. In sun-poor regions, this advantage outweights the increased cancer risk, and therefore humans in northern latitudes have evolved lighter skin. Dark-skinned people who live in sun-poor regions often lack vitamin D3 (one reason for the fortification of milk with vitamin D in some countries). People in sun-rich regions typically have an overabundance of vitamin D3.
The inuit are a special case: even though they live in an extremely sun-poor environment, they have retained their relatively dark skin. This can be explained by the fact that their special diet provides plenty of vitamin D.