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SciShow explains the genetics -- and physics -- behind why blue eyes are blue, and what the future may be for the trait. Spoiler alert: Blue eyes aren’t really blue! SciShow explains!

Hosted by: Hank Green
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Despite the apparent prevalence of the trait among swanky crooners and British spies and creepy old creatures who just hung onto the ring for too long, only about 8% of the world's human population has blue eyes. Here in the U.S. it's about twice that, but that's still a lot less than it was at the turn of the twentieth century when nearly half of all Americans had the trait. So what happened? Are blue-eyed humans going the way of the dodo?

Well, no. We cleared up that rumor about redheads going extinct; the same goes for blue eyes. While the trait is becoming more rare, it's unlikely it will disappear all together. Which is crazy when you consider that 10,000 years ago blue eyes didn't even exist. In fact, there's a good chance that blue-eyed people may all share one common ancestor. Studies over the past decade have actually traced the trait to a mutation that most likely arose among brown-eyed people in a single human in the Black Sea region of southeastern Europe between 6,000 and 10,000 years ago.

The mutation affected what's known as the OCA2 gene, which helps our bodies produce melanin, the brownish pigment that gives color to our hair and eyes and skin. The mutation created kind of a dimmer switch for the pigmentation, but it didn't affect the entire gene. Instead, it only affected the production of melanin in the iris, the ring structure around the eye that regulates how much light gets in.

The bulk of the iris is a thick layer of melanin-producing cells called the stroma. And the OCA2 mutation turns the production of melanin in the stroma way down, but the pigmentation still shows up elsewhere, like in the hair and skin and other parts of the iris. Basically, the mutation keeps the stroma from being brown.

So why blue then? Well, strictly speaking, blue eyes aren't actually blue. Instead, in people with this mutation, the stroma is full of nearly colorless cells. And when the light strikes them, they scatter the wavelengths back out, in a process similar to what makes smoke or fog look blue when light passes through them.

If there is some yellowish pigment in the stroma, then the blue light will combine with that to make green. Throw in a little bit of brown, and you have hazel.

So how did we go from having no blue-eyed people, to hundreds of millions of them in less than 10 millennia? No one's entirely sure why the trait spread so quickly through Europe. Some scientists think the mutation could have helped prevent certain eye disorders related to long, dark northern winters.

But another factors appears to be that, for whatever reason, lots of blue-eyed people simply mated with other blue-eyed people in the past, which kept the trait in circulation. Because for each of your genes, you have two different versions, called alleles, one from your father and one from your mother. If you have at least one dominant allele for a gene, that's the trait you have.

The blue eyes come from a recessive allele, which means if you inherit one allele for blue eyes and another for brown, you're going to have brown eyes. But you still carry the recessive blue allele, which can be passed on. Which means that that first person that had that blue-eyed mutation didn't have blue eyes. They had to pass that onto their children, and their children had to pass it onto their children, until eventually they came back together to make someone with two blue-eyed alleles.

When both parents have blue eyes, they both have two recessive blue-eyed genes, which means their children will also have blue eyes since there's no dominant gene to mask the recessive one. This is how you end up with Scandinavian countries that are 95% blue-eyed, and it also explains why the percentage of blue eye-ers is dropping in much of the western world.

Human populations are a lot more evenly spread throughout the world than they were 10,000 years ago, so more blue eyes are pairing up with brown eyes and producing brown-eyed offspring.

I wouldn't go so far as to call them endangered, but safe to say those blue-eyed singers and spies and Gollums will always be in the minority.

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