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The non-tanning 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 climates by encouraging higher levels of vitamin D production and also allows the individual to retain heat better than someone with darker skin. concluded that red hair is not the result of positive selection but of a lack of negative selection.
Cultural reactions have varied from ridicule to admiration; many common stereotypes exist regarding redheads and they are often portrayed as fiery-tempered.
When henna is used on hair it dyes the hair to different shades of red.
Emigration from Europe has multiplied the population of red haired humans in the Americas, Australia, New Zealand and South Africa. the largest population of redheads in the world, at 6 to 18 million, compared to approximately 420,000 in Ireland and 300,000 in Scotland.
Abd ar-Rahman I also had red hair, his mother being a Christian Berber slave.
Red hair is also found amongst the Ashkenazi Jewish populations.
Even if both parents do not have red hair themselves, both can be carriers for the gene and have a redheaded child.
Genetic studies of dizygotic (fraternal) twins indicate that the MC1R gene is not solely responsible for the red hair phenotype; unidentified modifier genes exist, making variance in the MC1R gene necessary, but not always sufficient, for red hair production.In Asia, genetic red hair is rare, but reddish-brown (auburn) hair can be found in the Levant (Syria, Lebanon, Jordan, Israel and Palestine), in Turkey, in Caucasia, in Northern Kazakhstan, among Uyghurs and among Indo-Iranians.The use of henna on hair and skin for various reasons occasionally occurs in Asia.The pigment pheomelanin gives red hair its distinctive color.Red hair has far more of the pigment pheomelanin than it has of the dark pigment eumelanin.The genetics of red hair, discovered in 1997, appear to be associated with the melanocortin-1 receptor (MC1R), which is found on chromosome 16.