Approximately 1% to 2% of the human population has red hair, the condition occuring more frequently (between 2% and 6% of the population) in northern and western Europeans, and their descendants. Red hair appears in people with two copies of a recessive gene on chromosome 4 which causes a change in the MC1R protein. It is associated with fair skin color, freckles, and sensitivity to ultraviolet light.
Several accounts by Greek writers detail redheaded people. A fragment by the Greek poet Xenophanes describes the Thracians as blue-eyed and red haired. The Greek historian Herodotus described the "Budini", probably Udmurts and Permyak Finns located on the Volga in what is modern-day Russia, as being predominantly redheaded. The Greek historian Dio Cassius described Boudica, the famous Celtic Queen of the Iceni, to: "be tall and terrifying in appearance ... a great mass of red hair ... over her shoulders."
The Roman Tacitus commented on the "red hair and large limbs of the inhabitants of Caledonia (Scotland)," which he linked with some red haired Gaulish tribes of Germanic and Belgic relation. Red hair has also been found in Asia, notably among the Tocharians who occupied the northwesternmost province of what is modern-day China. The 2nd millennium BC caucasian Tarim mummies in China were found with red and blonde hair and most likely were of European origin.
Today, red hair is most commonly found at the western fringes of Europe; it is associated particularly with the people of the British Isles. Scotland has the highest proportion of redheads, as 13 percent of the population has red hair and approximately 40 percent carries the recessive redhead gene. Ireland has the second highest percentage; as many as 10 percent of the Irish population have red, auburn, or strawberry blond hair. It is thought that up to 46 percent of the Irish population carries the recessive redhead gene.
In the United States, anywhere from two to six percent of the population is estimated to have red hair. This would give the U.S. the largest population of redheads in the world, at 6 to 18 million, compared to approximately 650,000 in Scotland and 420,000 in Ireland.
The pigment pheomelanin gives red hair its distinctive colour. Red hair has far more pheomelanin than other hair colors, but far less of the dark pigment eumelanin. The pale skin associated with red hair may be of advantage 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.
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. Due to this sensitivity many people have advised redheads to wear sunscreen.
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