The color of a human hair is determined by the type and concentration of a pigment called melanin. There are several types of melanin in hair. The concentration of one of the types of melanin, called eumelanin, gives the hair a black or brown color, and if this concentration is very low, the blond color. The concentration of another pigment, called pheomelanin, gives the red color.
The type and amount of melanin in hair is determined by many genes. The most studied gene that determines hair color in humans is called "MC1R". This gene provides instructions for the manufacture of a protein called "melanocortin receptor type 1", which is involved in the creation of melanin. This receptor controls the type of melanin produced by the melanocytes. When this receptor is activated, it triggers a series of chemical reactions within the melanocytes that stimulate the production of eumelanin.
If this receptor is not functional, the melanocytes produce pheomelanin instead of eumelanin. Many other genes also help regulate this process.
Most humans have two functional copies of the MC1R gene, inherited from each of their parents. These people have black or brown hair, due to the large amount of eumelanin present in the hair. It is estimated that more than 90 percent of human have brown or black hair!
Some people have variations of a copy of the MC1R gene in each cell who deactivates the gene. For these people, the production of eumelanin is lower, while the production of pheomelanin is higher, which gives them a light brown, blond or red color.
The color of the hair can change over time. And especially for people from European origin. For example, children with blond hair often have darker hair when they become teenagers. The researchers speculate that some hair pigment proteins are activated as children grow older. With age, our hair turns gray when the hair follicle loses its ability to produce melanin.