|It is "common knowledge" that redheads have hot tempers, but no one has ever
seen a controlled study that tested the correlation. So while some like to use
that excuse, it probably isn't a valid one.
However, there are a few things that are clearly associated with red hair. Redheads are generally pale, freckle easily, and tan poorly. Like anyone with fair skin, we are likely to sunburn without sunscreen, and we're at increased risk for the eventual development of skin cancer.
There are a few strange syndromes associated with red hair, but they are very rare. One of them is the Brittle Cornea Syndrome, which is seen very infrequently in Tunisian Jews. It is inherited in an autosomal recessive pattern (you need two mutant copies of the gene to be affected) and includes several apparently unrelated traits such as brittle corneas, blue sclera (the whites of the eyes are bluish), and hyperextensible joints. In some families the affected individuals always have red hair!
All of this should make you think about the genetics of hair color, which is a complex and not well understood subject. However, a few things are known about the genetics of red hair. One genetics textbook explained it this way: a person with red hair has genes for light colored hair (blond), and also possesses separate genes specifying red hair. This combination makes red hair -- redheads are blonds in disguise! Someone with dark hair might also have the red hair genes, but their dark hair genes overrule the red genes and so they don't have red hair. This would also explain why dark-haired parents can sometimes have redhead children.
Nothing else I have read directly contradicts this model, but it looks like it is more complex than that. In 1987, a Danish group's research suggested that there is a gene on Chromosome 4 that is involved in red hair color. More recently, a group at the University of Newcastle upon Tyne, UK, found that variations in a specific gene that encodes a hormone receptor are often associated with red hair.
The gene that they are looking at is the MC1R gene located on chromosome 8. They found that redheads often have mutations in this gene. The MC1R gene is involved in telling certain cells to make a brown-black type of melanin called eumelanin. The lack of this gene would result in decreased production of eumelanin. People with a mutation in the MC1R gene would instead make another type of melanin called pheomelanin. Pheomelanin is a red-brown pigment and provides less UV protection than eumelanin does.
This all starts to make sense if you examine the hair of redheads. Unlike brown or black hair, the pigment of red hair is almost all pheomelanin, with very little eumelanin! It can't possibly be that simple though, because there are plenty of redheads who don't have mutations in the MC1R gene. Other genes, like the unknown gene on chromosome 4, must also play roles in determining red hair.
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