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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. Melanocytes, which are cells that produce pigment in skin and hair, use the MC1R receptor to recognize and respond to melanocyte-stimulating hormone from 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. 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. One study, however, reports a link between red hair and a higher rate of bruising.