Hair Color
For hair colorants, see Hair coloring.
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.
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. 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).
Natural hair colors
Natural hair color can be black, brown, blond, or red, depending on a person's ethnic origins. It is characterized by higher levels of eumelanin and lower levels of pheomelanin.
Strawberry blond, a mixture of blond and red hair only (thought to have originated in Celtic and Scandinavian countries) is a much rarer type containing the most amounts of phaeomelanin.
Blond hair can have almost any proportion of pheomelanin and eumelanin, but both only in small amounts. More pheomelanin creates a more golden blond color, and more eumelanin creates an ash blond.
Blond hair is common in many European peoples, but rare among peoples of non-European origin. Many children born with blond hair develop darker hair as they age.
In some cases, grey hair may be caused by thyroid deficiencies or a deficiency of B12.
The Journal of Investigative Dermatology published a study in 2005 that claimed white people will begin to gray in their mid-thirties and Asian people begin graying in their late thirties, but most black people can retain their original hair color until their mid-forties. Many blond, strawberry blond, light brown, or red haired infants experience this.
Changes in hair color typically occur naturally as people age, eventually turning the hair gray and then white.
More than 40 percent of Americans have some gray hair by age 40, but white hairs can appear as early as childhood. The age at which graying begins seems almost entirely due to genetics.
Sometimes people are born with gray hair because they inherit the trait.
Two genes appear to be responsible for the process of graying, Bcl2 and Bcl-w. The change in hair color occurs when melanin ceases to be produced in the hair root and new hairs grow in without pigment.
The stem cells at the base of hair follicles produce melanocytes, the cells that produce and store pigment in hair and skin. The death of the melanocyte stem cells causes the onset of graying.
Other medical conditions affecting hair color
Albinism is a genetic abnormality in which little pigment is found in human hair, eyes or skin.
The hair is white or pale blond.
Vitiligo is a patchy loss of hair and skin color that may occur as the result of an auto-immune disease.
Malnutrition is also known to cause hair to become lighter, thinner, and more brittle. The condition is reversible with proper nutrition.
Werner syndrome and pernicious anemia can also cause premature graying.
A recent study demonstrated that people 50–70 years of age with dark eyebrows but gray hair are significantly more likely to have type II diabetes than those with both gray eyebrows and hair.
Artificial factors affecting hair color
A 1996 British Medical Journal study conducted by J.G.
Much remains to be learned about the physiology of human graying.
There are no special diets, nutritional supplements, vitamins, nor proteins that have been proven to slow, stop, or in any way affect the graying process, although many have been marketed over the years. French scientists treating leukemia patients with a new cancer drug noted an unexpected side effect: some of the patients' hair color was restored to their pre-gray color.
Changes in hair color after death
The hair color of mummies or buried bodies can change.
A one-color permanent dye creates a flat, uniform color across the whole head, which can look unnatural and harsh, especially in a dark shade. To combat this, the modern trend is to use multiple colors - usually one color as a base with added highlights or lowlights in other shades.
Semi-permanent color washes out over a period of time – typically four to six weeks, so root regrowth is less noticeable.
However, this means that gray and white hair will not dye to the same color as the rest of the head (in fact, some white hair will not absorb the color at all). A few gray and white hairs will blend in sufficiently not to be noticeable, but as they become more widespread, there will come a point where a semi-permanent alone will not be enough.