History

Genetics and biochemistry of hair color

There are two types (three subtypes) of pigment that give hair its color: eumelanin and phaeomelanin. Eumelanin is black and brown while phaeomelanin is red. The amount of eumelanin in hair determines the darkness of its color. A low concentration of brown eumelanin in the hair will make it blonde, whereas more brown eumelanin will give it a brown color. Much higher amounts of black eumelanin will result in black hair, and a low concentration of black eumelanin in the hair will make it gray. All humans have phaeomelanin in their hair.
Phaeomelanin is more chemically stable than black eumelanin, but less chemically stable than brown eumelanin, so it breaks down more slowly when oxidized. This is the reason bleach will cause darker hair to turn reddish-brown during the artificial coloring process. As the phaeomelanin continues to break down, the hair will gradually become orange and later yellow, and then white.

Light hair map ("light" hair refers to blonde, red, and light brown). The yellow represents 80%+ light hair, light orange is 50-79% light hair, light brown is 20-49% light hair, dark brown is 1-19% light hair, and black represents no presence of light hair in the indigenous population.
The genetics of hair colors are not yet firmly established. According to one theory, at least two gene pairs control human hair color. One gene, which is a brown/blonde pair, has a dominant brown allele and a recessive blonde allele. A person with a brown allele will have brown hair; a person with no brown alleles will be blonde. This also explains why two brown-haired parents can produce a blonde-haired child. The other gene pair is a not-red/red pair, where the not-red allele (which suppresses production of pheomelanin) is dominant and the allele for red hair is recessive. Since the two gene pairs both govern hair color, a person with two copies of the red-haired allele will have red hair, but it will be either auburn or bright reddish orange depending upon whether the first gene pair gives brown or blonde hair, respectively.
The two-gene model does not account for all possible shades of brown, blonde, or red (for example, platinum blonde versus dark blonde/light brown), nor does it explain why hair color sometimes darkens with age. Several gene pairs control the light versus dark hair color in a cumulative effect. Therefore, the more of these that are dominant, the darker the hair will be. A person's genotype for a multifactorial trait can interact with environment to produce varying phenotypes (see quantitative trait locus).

Article source From Wikipedia, the free encyclopedia