Red hair occurs on approximately 1–2% of the human population. It occurs more frequently (2–6%) in people of northern or western European ancestry, and less frequently in other populations. Red hair appears in people with two copies of a recessive gene on chromosome 16 which causes a mutation in the MC1R protein.
Red hair varies from a deep burgundy through burnt orange to bright copper. It is characterized by high levels of the reddish pigment pheomelanin and relatively low levels of the dark pigment eumelanin. The term redhead (originally redd hede) has been in use since at least 1510. It is associated with fair skin color, lighter eye color (gray, blue, green, and hazel), freckles, and sensitivity to ultraviolet light.
Cultural reactions have varied from ridicule to admiration; many common stereotypes exist regarding redheads and they are often portrayed as fiery-tempered.
Red hair is the rarest natural hair color in humans. The pale skin associated with red hair may have been advantageous in far-northern climates where sunlight is scarce. Studies by Bodmer and Cavalli-Sforza (1976) hypothesized that lighter skin pigmentation prevents rickets in colder latitudes by encouraging higher levels of Vitamin D production and also allows the individual to retain heat better than someone with darker skin.[26] In 2000, Harding et al. concluded that red hair was not the result of positive selection and instead proposed that it occurs because of a lack of negative selection. In Africa, for example, red hair is selected against because high levels of sun would be harmful to fair skin. However, in Northern Europe this does not happen, so redheads come about through genetic drift.[23] Rees (2004) suggested that the vividness and rarity of red hair may lead to its becoming desirable in a partner and therefore it could become more common through sexual selection.[27]
Estimates on the original occurrence of the currently active gene for red hair vary from 20,000 to 100,000 years ago.[28][29]
A DNA study has concluded that some Neanderthals also had red hair, although the mutation responsible for this differs from that which causes red hair in modern humans.[30]
Melanin in the skin aids UV tolerance through suntanning, but fair-skinned persons lack the levels of melanin needed to prevent UV-induced DNA-damage. Studies have shown that red hair alleles in MC1R increase freckling and decrease tanning ability.[33] It has been found that Europeans who are heterozygous for red hair exhibit increased sensitivity to UV radiation.[27]
Red hair and its relationship to UV sensitivity are of interest to many melanoma researchers. Sunshine can both be good and bad for a person's health and the different alleles on MC1R represent these adaptations. It also has been shown that individuals with pale skin are highly susceptible to a variety of skin cancers such as melanoma, basal cell carcinoma, and squamous cell carcinoma.[34][35]
Two studies have demonstrated that people with red hair have different sensitivity to pain compared to people with other hair colors. One study found that people with red hair are more sensitive to thermal pain (associated with naturally occurring low vitamin K levels),[36] while another study concluded that redheads are less sensitive to pain from multiple modalities, including noxious stimuli such as electrically induced pain.[37][38][39]
Researchers have found that people with red hair require greater amounts of anesthetic.[40] Other research publications have concluded that women with naturally red hair require less of the painkiller pentazocine than do either women of other hair colors or men of any hair color. A study showed women with red hair had a greater analgesic response to that particular pain medication than men.[41] A follow-up study by the same group showed that men and women with red hair had a greater analgesic response to morphine-6-glucuronide.[39]
The unexpected relationship of hair color to pain tolerance appears to be because redheads have a mutation in a hormone receptor that can apparently respond to at least two different hormones: the skin pigmentation hormone melanocyte-stimulating hormone, and the pain relieving hormone known as endorphins. (These hormones are both derived from the same precursor molecule, POMC, and are structurally similar.) Specifically, redheads have a mutated MC1R gene, which produces a mutated MC1R receptor, also known as the melanocortin-1 receptor.[42] Melanocytes, which are cells that produce pigment in skin and hair, use the MC1R receptor to recognize and respond to melanocyte-stimulating hormonefrom the anterior pituitary gland. Melanocyte-stimulating hormone normally stimulates melanocytes to make black eumelanin, but if the melanocytes have a mutated MC1R receptor, they will make reddish pheomelanin instead. The MC1R receptor also occurs in the brain, where it is one of a large set of POMC-related receptors that are apparently involved not only in responding to MSH, but also in responses to endorphins and possibly other POMC-derived hormones.[42]Though the details are not clearly understood, it appears that there is some "cross talk" between the POMC hormones that may explain the link between red hair and pain tolerance.
There is little or no evidence to support the belief that people with red hair have a higher chance than people with other hair colors to hemorrhage or suffer other bleeding complications.[43][44] One study, however, reports a link between red hair and a higher rate of bruising.[44]