Human hair color is the pigmentation of human 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. The tone of the hair is dependent on the ratio of black or brown eumelanin to yellow or red pheomelanin. 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 on the same person. Some hair colors are associated with some ethnic groups due to observed higher frequency of particular hair color within their geographical region, e.g. straight dark hair amongst East Asians, Southeast Asians, Polynesians and Native Americans, a large variety of dark, fair, curly, straight, wavy and bushy hair amongst Europeans, West Asians and North Africans, curly, dark, and uniquely helical hair with Sub Saharan Africans, whilst gray, white or "silver" hair is often associated with age.
The full genetic basis of hair color is complex and not fully understood.[1] Regulatory DNA is believed to be closely involved in pigmentation in humans in general,[2] and a 2011 study by Branicki et al. identified 13 DNA variations across 11 different genes that could be used to predict hair color.[3]
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Two types of pigment give hair its color, black-brown eumelanin and reddish-brown/reddish-yellow[4] pheomelanin, synthesized by melanocytes.[5] Inside the melanocytes, tyrosine is converted into L-DOPA and then L-dopaquinone, which in turn is formed into pheomelanin or eumelanin.[6]
Different hair color phenotypes arise primarily as a result of varying ratios of these two pigments in the human population,[5] although Europeans show the greatest range in pigmentation overall.[7] In addition, other genetic and environmental factors can affect hair color in humans; for instance, mutations in the melanocortin 1 receptor (MC1R) gene can lead to red or auburn hair,[5] and exposure to ultraviolet radiation can damage hair and alter its pigmentation.[8] Ultraviolet radiation (UV radiation) triggers greater synthesis of several compounds, including pro-opiomelanocortin (POMC), -MSH, and ACTH, the result being increased eumelanin production.[6] UV radiation most commonly comes from the sun, and thus populations from places closer to the equator tend to have darker hair,[6] because eumelanin is generally more photoprotective than pheomelanin.[4]
Pheomelanin colors hair orange and red. Eumelanin, which has two subtypes of black or brown, determines the darkness of the hair color;[4] more black eumelanin leads to blacker hair, and more brown eumelanin to browner hair.[6] All human hair has some amount of both pigments.[9] Over 95% of melanin content in black and brown hair is eumelanin.[9] Pheomelanin is generally found in elevated concentrations in blond and red hair,[4] representing about one-third of total melanin content.[9] If there is no black eumelanin, the result is strawberry blond.[6] Blond hair results from small amounts of brown eumelanin with no black eumelanin.[6]
Brown hair is the second most common human hair color, after black. 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 girls or women of European, West Asian or North African descent are often known as brunettes.
Chestnut hair is a hair color which is a reddish shade of brown hair. In contrast to auburn hair, the reddish shade of chestnut is darker. Chestnut hair is common among the native peoples of Northern, Central, Western, and Eastern Europe and is also found in Asia Minor, West Asia and North Africa.
Auburn hair ranges along a spectrum of light to dark red-brown shades. The chemicals which cause auburn hair are eumelanin (brown) and pheomelanin (red), with a higher proportion of red-causing pheomelanin than is found in average brown hair. It is most commonly found in individuals of Northern and Western European descent, but is extant in West and Central Asia and North Africa also. It can also be the result of a mutation in the melanocortin 1 receptor gene.[5]
Blond (sometimes blonde for women) hair ranges from pale white (platinum blond) to dark gold blond. Strawberry blond, a mixture of blond and red hair, is a much rarer type containing the most pheomelanin.[citation needed]Blond hair can have almost any proportion of pheomelanin and eumelanin, but has only small amounts of both. More pheomelanin creates a more golden or strawberry blond color, and more eumelanin creates an ash or sandy blond color. Blond hair is most commonly found in Northern and Northeastern Europeans and their descendants but can be found spread around most of Europe and also among West Asians and North Africans at lower frequencies. Studies in 2012 showed that naturally blond hair of Melanesians is caused by a recessive mutation in tyrosinase-related protein 1 (TYRP1). In the Solomon Islands, 26% of the population carry the gene; however, it is absent outside of Oceania.[16]
Gray or white hair is not caused by a true gray or white pigment, but is due to a lack of pigmentation and melanin. The clear hairs appear as gray or white because of the way light is reflected from the hairs. Gray hair color typically occurs naturally as people age (see aging or achromotrichia below).
Europeans often begin to grow gray hairs in their mid-30s while Asians begin graying in their late 30s, but most Africans retain their original hair color until their mid-40s.Permanently white hair starting in childhood can be genetically inherited, but unlike albinism, there are no negative medical implications. The trait follows X-linked recessive inheritance, and so is more common in men, and women can be carriers without being affected.[20][21]
Graying is a gradual process; according to a study by L'Oreal, overall, of those between 45 and 65 years old, 74% had some gray hair, covering an average of 27% of their head, and approximately 1 in 10 people had no gray hairs even after the age of 60.[22][23]
Children born with some hair colors may find it gradually darkens as they grow. Many blond, light brown, or red haired infants experience this. This is caused by genes being turned on and off during early childhood and puberty.[26]
Changes in hair color typically occur naturally as people age, eventually turning the hair gray and then white. This is called achromotrichia. Achromotrichia normally begins in the early to mid-twenties in men and late twenties in women. More than 60 percent of Americans have some gray hair by age 40. The age at which graying begins seems almost entirely due to genetics. Sometimes people are born with gray hair because they inherit the trait.[27]
In non-balding individuals, hair may grow faster once it turns gray.[29] Unlike in the skin where pigment production is continuous, melanogenesis in the hair is closely associated with stages of the hair cycle. Hair is actively pigmented in the anagen phase and is "turned off" during the catagen phase, and absent during telogen.[30] Thus, a single hair cannot be white on the root side, and colored on the terminal side.
Several genes appear to be responsible for the process of graying. Bcl2 and Bcl-w[31] were the first two discovered, then in 2016, the IRF4 (interferon regulatory factor 4) gene was announced after a study of 6,000 people living in five Latin American countries. However, it found that environmental factors controlled about 70% of cases of hair graying.[32]
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. It remains unclear why the stem cells of one hair follicle may fail to activate well over a decade before those in adjacent follicles less than a millimeter apart.
Graying of hair may be triggered by the accumulation of hydrogen peroxide and abnormally low levels of the enzyme catalase, which breaks down hydrogen peroxide and relieves oxidative stress in patients with vitiligo. Since vitiligo can cause eyelashes to turn white, the same process is believed to be involved in hair on the head (and elsewhere) due to aging.[33][unreliable source?]
The anti-cancer drug imatinib has recently been shown to reverse the graying process.[34] However, it is expensive and has potentially severe and deadly side effects, so it is not practical to use to alter a person's hair color. Nevertheless, if the mechanism of action of imatinib on melanocyte stem cells can be discovered, it is possible that a safer and less expensive substitute drug might someday be developed. It is not yet known whether imatinib has an effect on catalase, or if its reversal of the graying process is due to something else.[35]
Anecdotes report that stress, both chronic and acute, may induce achromotrichia earlier in individuals than it otherwise would have.[36] Proponents point to survivors of disasters, such as Titanic survivor Harold Bride, prisoner of war John McCain, or high-level politicians such as Bill Clinton or Barack Obama. There is some evidence for chronic stress causing premature achromotrichia,[37] but no definite link has been established. It is known that the stress hormone cortisol accumulates in human hair over time, but whether this has any effect on hair color has not yet been resolved.[38] A 2020 paper, published in the journal Nature reported that stress can cause hair to lose its pigment. An overactive immune response can destroy melanocytes and melanocyte stem cells in black-haired rats. When intentionally subjecting them to panic, they bleached their coat. The next time the rats' coat grew, there were no melanocyte stem cells in these damaged follicles, so white hairs sprouted, and the color loss was permanent.[39] 2351a5e196
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