Human hair color

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Hair color is the result of pigmentation due to the chemicals eumelanin and phaeomelanin. In general, the more melanin present, the darker the hair color; the less melanin, the lighter the hair color. Considerable differences in color and texture exist between individuals of similar ethnicity. Immigration and global travel have greatly increased the diversity of hair characteristics among many countries. A person's hair color may also change over time and may be more than one color at a time.

[edit] 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).

[edit] Natural hair colors

Natural hair color is black, brown, blonde, or red, depending on a person's ethnic origins. Hair color is genetically associated with certain skin tones and eye colors.

Black hair	Brown hair	Blond hair	Red hair
Gray hair	White hair

[edit] Black hair

Main article: Black hair

Black hair is the darkest and most common color of human hair, and is found in people of all ethnicities. It has large amounts of eumelanin and is less dense than other hair colors. However, it may only be a very dark brown.^[1]

[edit] Brown hair

Main article: Brown hair

Brown hair is the second most common hair color after black and is also found mostly among people of European ancestry. It is characterized by higher levels of the dark pigment eumelanin and lower levels of the pale pigment phaeomelanin. 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 also known as brunettes, although this term is usually applied only to females and rarely used to describe males with brown hair. Brown hair is most commonly found in peoples with european ancestry.

[edit] Blond hair

Main article: Blond

Blond hair ranges from nearly white (platinum blond, tow-haired) to a dark golden blond. Strawberry blond, a mixture of blond and red hair found predominantly in Ireland, is an especially rare type containing the most phaeomelanin. Blond hair can have almost any proportions of phaeomelanin and eumelanin, but both only in small amounts. More phaeomelanin creates a more golden blond color, and more eumelanin creates an ash blond. Blond hair is common in many European peoples, but rare among others. Many children born with blonde hair may develop darker hair as they age.^{[citation needed]}

[edit] Red hair

Main article: Red hair

Red hair ranges from vivid strawberry shades to deep auburn and burgundy. It is caused by a variation in the Mc1r gene and believed to be recessive.^[2] Red hair has the highest amounts of phaeomelanin and usually low levels of eumelanin.^{[citation needed]}

[edit] Gray hair

Gray hair color typically occurs naturally as people age (see "Effects of aging on hair color", below). The same can be said for white hair. In some cases, gray hair may instead be caused by a deficiency of B12 or a thyroid imbalance.^[3]

[edit] Conditions affecting hair color

[edit] Effects of aging on hair color

A gray-haired man

A change in hair color typically occurs naturally as people age, usually turning their hair to 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 to be almost entirely based on genetics. Sometimes people are born with gray hair because they inherit the trait genetically. Some people use hair dye to disguise the amount of gray in their hair.

This occurs in the younger population also. Children born with a certain color may find that it gradually darkens as they grow. Many blond, strawberry blond, light brown, or red haired infants experience this.

Two genes appear to be responsible for the process of graying, Bcl2 and Bcl-w. The change in hair color is caused 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 are responsible for producing melanocytes, the cells that produce and store pigment in hair and skin. The death of the melanocyte stem cells causes the onset of graying.^[4]

[edit] Other medical conditions affecting hair color

Albinism is a genetic abnormality in which no pigment is found in human hair, eyes or skin. This results in gray, blue, or red eyes. The skin is pale and 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. Dark hair may thus turn reddish or blondish due to the decreased production of melanin. 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.^[5]

[edit] Artificial factors affecting hair color

A 1996 British Medical Journal study conducted by J.G. Mosley, MD found that tobacco smoking may cause premature graying. Smokers were found to be four times more likely to begin graying prematurely, compared to nonsmokers in the study.^[6]

Gray hair may temporarily darken after inflammatory processes, after electron-beam-induced alopecia, and after some chemotherapy regimens. Much remains to be learned about the physiology of human graying.^[7]

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. This may change in the near future. 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.^[8]

[edit] Changes in hair color after death

The hair color of mummies or buried bodies can change over large time periods. Hair contains a mixture of black-brown-yellow eumelanin and red pheomelanin. Eumelanin is less chemically stable than pheomelanin and breaks down faster when oxidized. It is for this reason that Egyptian mummies have reddish hair. The color of hair changes faster under extreme conditions. It changes more slowly under dry oxidizing conditions (such as in burials in sand or in ice) than under wet reducing conditions (such as burials in wood or plaster coffins).^[9]

[edit] Hair coloring

A hairdresser colors a client's hair.

Main article: Hair coloring

The process of changing a person's hair color can be done by a chemical process known as hair coloring. Hair coloring can be permanent or temporary and the lasting effects are determined, in part, by the texture of the individual's hair.

The use of chemical lighteners, such as bleach, is one way hair is lightened or "highlighted". This type of hair coloring is always permanent because it involves the removal of natural pigment, which never returns. Semi-permanent hair color can darken or change the tonal value of the hair, but cannot lighten the hair and can usually be completely washed away after several shampoos. Semi-permanent hair color is only a deposit of hair color. This hair color is used to darken natural hair color. "Rinses" are a form of temporary hair color that are usually applied to hair during a shampoo. Their effects usually only last until the hair is shampooed or rinsed. Permanent hair color is probably the most-utilized because of its ability to affect the hair in level (lightness or darkness) as well as tone, but it comes with a unique set of potential problems, such as the need to frequently re-apply, unwanted fading and hot roots.

[edit] See also

• Eye color
• Human skin color
• List of Mendelian traits in humans

[edit] Footnotes

[edit] External links

• A chart of hair colors
• The return of natural color gray hair

v • d • e Human hair By area Head hair · Androgenic hair (Facial hair · Chest hair · Underarm hair · Abdominal hair · Pubic hair · Leg hair) By type Vellus hair · Terminal hair · Lanugo Related topics Human hair color · Hairstyle · Hypertrichosis · Trichophilia · Hair removal · Hair loss