hair color

Hair color is the pigmentation of hair follicles due to two types of melanin, eumelanin and pheomelanin. Generally, if more melanin is present, the color of the hair is darker; if less melanin is present, the hair is lighter. Levels of melanin can vary over time causing a person's hair color to change, and it is possible to have hair follicles of more than one color.
Particular hair colors are associated with ethnic groups. The shades of human hair color are assessed by Fischer–Saller scale. The Fischer–Saller scale, named after Eugen Fischer and Karl Saller, is used in physical anthropology and medicine to determine the shades of hair color. The scale uses the following designations: A (light blond), B to E (blond), F to L (blond), M to O (dark blond), P to T (brown), U to Y (dark brown/black) and Roman numerals I to IV (red) and V to VI (red blond).See also Martin–Schultz scale.
Two types of pigment give hair its color: eumelanin and pheomelanin. Pheomelanin colors hair red. Eumelanin, which has two subtypes of black or brown, determines the darkness of the hair color. A low concentration of brown eumelanin results in blond hair, whereas a higher concentration of brown eumelanin will color the hair brown. High amounts of black eumelanin result in black hair, while low concentrations give gray hair. All humans have some pheomelanin in their hair.
Pheomelanin is more chemically stable than black eumelanin, but less chemically stable than brown eumelanin, so it breaks down more slowly when oxidized. This is why bleach gives darker hair a reddish tinge during the artificial coloring process. As the pheomelanin continues to break down, the hair will gradually become orange, then yellow, and finally white.

The genetics of hair colors are not yet firmly established. According to one theory, at least two gene pairs control human hair color.

One phenotype (brown/blond) has a dominant brown allele and a recessive blond allele. A person with a brown allele will have brown hair; a person with no brown alleles will be blond. This explains why two brown-haired parents can produce a blond-haired child.
The other gene pair is a non-red/red pair, where the not-red allele (which suppresses production of pheomelanin) is dominant and the allele for red hair is recessive. 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 on whether the first gene pair gives brown or blond hair, respectively.

The two-gene model does not account for all possible shades of brown, blond, or red (for example, platinum blond versus dark blonde/light brown), nor does it explain why hair color sometimes darkens as a person ages. Several gene pairs control the light versus dark hair color in a cumulative effect. A person's genotype for a multifactorial trait can interact with environment to produce varying phenotypes (see quantitative trait locus).
Brown hair is characterized by higher levels of eumelanin and lower levels of pheomelanin. Of the two types of eumelanin (black and brown), brown-haired people have brown eumelanin; they also usually have medium-thick strands of hair. Brown-haired people are often known as brunettes/brunets.