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  • Hey smart pupils, Joe here.

  • William Shakespeare called the eyes thewindow to the soul.”

  • But it turns out they're actually just a window that focuses photons onto a light-sensing

  • tissue called the retina.

  • And these are mine.

  • They're brownish.

  • With a sort of hazel ring going on?

  • I'm honestly not even really sure what hazel is, but let's go with it.

  • And these are the eyes of some really cool and popular YouTubers who kindly let me stick

  • my macro lens in their face when we were hanging outbecause I'm super normal in social

  • situations!

  • Can you tell who these belong to?

  • If not, don't feel bad, I mean when was the last time you just staaaared at someone's

  • eyeball from six inches away?

  • It's kinda weird.

  • In fact, looking at eyes looking back at me is starting to creep me out

  • Much better.

  • Our eyes are just as unique as our fingerprints, like tiny galaxies, full of shapes and patterns

  • and colorswhere do these eye colors come from?

  • Don't blink, here comes the science.

  • [OPEN]

  • What color are your eyes?

  • Anthropologists use an official scale to classify people's eye colors from around the world.

  • Tag yourself

  • I'm a 10.

  • I've always wanted to say that.

  • That little colored bit on the front is called theiris”.

  • It's named for the Greek goddess of the rainbow, but this entire kaleidoscope of colors

  • is actually made from just one color.

  • The blues, the greens, the brownsare all a trick of physics.

  • If you pulled out your irises and tried to look at the pigment molecules inside, you

  • couldn't, because you'd be blind.

  • But they would all be brown!

  • The iris contains cells that contain a pigment called melanin and there's no blue melanin.

  • Onlyearthy tones”.

  • In every color of eye, the back, most inside layer of the iris is densely packed with dark

  • melanin.

  • But that's totally covered by a meshy front layer, and that's where things vary from

  • person to person.

  • If you have brown eyes, the cells in the meshy front of your iris are full of pigment.

  • If you have blue, or blue-grey, there's not much pigment in that meshy layer at all

  • The other eye colors fall somewhere in the middle.

  • So how can you have blue eyes when there's nothing blue inside them?

  • Let me show you this quick experiment.

  • If I shine a light through plain water, the beam is almost invisible.

  • But when we add some milk, the beam becomes visible and it has a bluish tint.

  • It's called the Tyndall effect.

  • There's tiny particles of milk suspended in the water, the same way there's tiny

  • packets of pigment spread out inside blue eyes.

  • Even though those particles aren't blue, they're scattering shorter blue wavelengths

  • of light away, while redder wavelengths pass through.

  • It's also why smoke looks blue, and why the sky is blue

  • though in the case of the sky, light is scattering off air molecules and not dust

  • particleshey, this is science.

  • Details matter.

  • So how's this happening inside the eye?

  • In people with blue eyes, those scattered pigment particles bounce blue light back out

  • of the iris due to the Tyndall effect, and the iris absorbs the red light.

  • So the eye appears blue even though there's nothing blue in it.

  • The denser those little pigment specks, the more brown it looks.

  • Light-colored eyes like green, blue, and grey, aren't truly green, blue, or grey.

  • They're just ... less and less brown.

  • For most of the human population brown eyes dominate, but we find pockets of lighter-eyed

  • people throughout the world.

  • The mutation that originally caused blue eyes in European populations is at least 6,000

  • years old, but it could be even older - maybe originating in Africa more than 10,000 years

  • ago.

  • Speaking of which, how is eye color inherited, anyway?

  • Traits come, in part, from genes: little bits of DNA, passed down from parents.

  • In grade school, maybe you heard that one gene determines eye color, and the brown-eyed

  • trait always dominates over the blue-eyed trait.

  • Or that blue-eyed parents can't have brown-eyed kids?

  • Well that's all wrong.

  • This kid, came from these parents.

  • Eye color genetics is nowhere near as simple as we've been taught, and you're not secretly

  • adoptedwell, maybe you are, but you wouldn't know because of your eyes.

  • Maybe you've heard of this monk, Gregor Mendel, who played with peas a lot back in

  • the 1800s?

  • Most our old explanations about eye color genetics came from his ideas, but more recently

  • we've discovered that most traits don't fit neatly into Mendel's little squares

  • because they involve several genes interacting together.

  • Like, we have this gene.

  • How strongly it's turned on determines how much pigment cells in our eyes make.

  • What version of this gene you have is responsible for around 75 percent of having blue vs. brown

  • eyes.

  • BUT, another gene that lives close by can interfere with or even switch that gene off.

  • So even if you have a “browntrait here, you can end up with lighter eyes.

  • And that's just two genes!

  • In all, we've IDed at least 10 genes that influence eye color, and there are probably

  • a lot more.

  • Like height or intelligence, it's more like a genetic symphony, all playing with one another.

  • No one's eyes can really be described by a single word, they fall on a continuous rainbow

  • from brown to blue, and a lot in between.

  • Turns out, your eye color is one of the most beautiful and unique things about you.

  • What's an interesting way to describe YOUR eyes?

  • Eye'll be looking for answers in the comments.

  • Stay tuned for an even worse eye pun, but first

  • I want to tell you about the new PBS Digital Studios series Say It Loud, a celebration

  • of Black history and culture, and its impact on how we live today.

  • It's hosted by Evelyn from the Internets and Azie Dungey, they give you a hilarious

  • take on identity and pop culture, and I recently joined them for an episode about DNA ancestry

  • tests.

  • Check out Say It Loud at the link in the description below.

  • And finally, you know what they say about eye punscould they BE any cornea?

  • Stay curious!

Hey smart pupils, Joe here.

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B1 US eye pigment eye color brown iris gene

Is Your Eye Color Real?

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    minami.kuo posted on 2019/12/16
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