There's a lot of natural variation in the color of human hair. What's the physical explanation for the difference?
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Sometimes, you’re right in the middle of cleaning out the drain in the shower and you start pondering questions like, “Why is my hair a different color from my mom’s hair, or my neighbor’s hair, or my roommate’s disgusting, soggy, three-foot-long, wolf-tail drain-wad? What’s the real difference between blonde hair, black hair, red hair, and everything in between?”
The main structural ingredient in human hair is a protein called keratin. It’s what your hair and fingernails are made of, but also what’s behind the silky sheen of wool, bear claws and horse hooves. Mmm, don’t you just want to run your fingers through those hooves?
But keratin on its own is not very colorful. And if all humans had in our hair was keratin, we’d look like 18th-century French aristocrats in powdered wigs, because we’d all have the same sort of white, colorless hair.
But, keratin is not the only ingredient in human hair. To create natural color, you need to add pigment. This is done by cells in the skin called melanocytes. These melanocytes create the natural pigment known as melanin and deliver it to the cells that create the keratin for your hair. This melanin comes in two varieties: eumelanin and pheomelanin.
Eumelanin is a dark pigment that gives hair a brown or black color. Pheomelanin is a lighter pigment that gives hair a red, orange or yellow-ish color. Both of these are present in varying degrees – a person might have a little of each, or a lot of one and almost none of the other. So someone with black or dark brown hair probably has a lot of eumelanin. A redhead has a lot of pheomelanin, and blondes don’t have very much of either one.
So what happens when we get older and start to “go gray”? You can probably guess: over time, melanocytes start to die off, and any new hair that grows has less pigment, so it looks gray or white.
But! you might be asking: “What determines the eumelanin to pheomelanin mixture to begin with? Who writes that recipe?”
Primarily, it’s your genes.
For example: the melanocortin 1 receptor, or MC1R, gene. When the protein associated with this gene is active in melanocytes, it stimulates them to make eumelanin – the pigment that colors black or brown hair. When MC1R is not active in the melanocyte cells, they make mostly pheomelanin instead, and Hello Weasleys.
But! The MC1R gene is not the only genetic factor that controls hair color – like most of your traits, hair color is actually affected by more than one genetic variable.
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