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Hey smart pupils, Joe here.
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William Shakespeare called the eyes the “window to the soul.”
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But it turns out they're actually just a window that focuses photons onto a light-sensing
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tissue called the retina.
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And these are mine.
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They're brown… ish.
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With a sort of hazel ring going on?
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I'm honestly not even really sure what hazel is, but let's go with it.
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And these are the eyes of some really cool and popular YouTubers who kindly let me stick
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my macro lens in their face when we were hanging out… because I'm super normal in social
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situations!
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Can you tell who these belong to?
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If not, don't feel bad, I mean when was the last time you just staaaared at someone's
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eyeball from six inches away?
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It's kinda weird.
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In fact, looking at eyes looking back at me is starting to creep me out…
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Much better.
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Our eyes are just as unique as our fingerprints, like tiny galaxies, full of shapes and patterns
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and colors… where do these eye colors come from?
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Don't blink, here comes the science.
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[OPEN]
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What color are your eyes?
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Anthropologists use an official scale to classify people's eye colors from around the world.
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Tag yourself…
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I'm a 10.
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I've always wanted to say that.
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That little colored bit on the front is called the “iris”.
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It's named for the Greek goddess of the rainbow, but this entire kaleidoscope of colors
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is actually made from just one color.
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The blues, the greens, the browns… are all a trick of physics.
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If you pulled out your irises and tried to look at the pigment molecules inside, you
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couldn't, because you'd be blind.
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But they would all be brown!
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The iris contains cells that contain a pigment called melanin and there's no blue melanin.
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Only “earthy tones”.
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In every color of eye, the back, most inside layer of the iris is densely packed with dark
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melanin.
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But that's totally covered by a meshy front layer, and that's where things vary from
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person to person.
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If you have brown eyes, the cells in the meshy front of your iris are full of pigment.
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If you have blue, or blue-grey, there's not much pigment in that meshy layer at all
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The other eye colors fall somewhere in the middle.
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So how can you have blue eyes when there's nothing blue inside them?
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Let me show you this quick experiment.
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If I shine a light through plain water, the beam is almost invisible.
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But when we add some milk, the beam becomes visible and it has a bluish tint.
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It's called the Tyndall effect.
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There's tiny particles of milk suspended in the water, the same way there's tiny
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packets of pigment spread out inside blue eyes.
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Even though those particles aren't blue, they're scattering shorter blue wavelengths
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of light away, while redder wavelengths pass through.
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It's also why smoke looks blue, and why the sky is blue…
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…though in the case of the sky, light is scattering off air molecules and not dust
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particles… hey, this is science.
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Details matter.
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So how's this happening inside the eye?
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In people with blue eyes, those scattered pigment particles bounce blue light back out
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of the iris due to the Tyndall effect, and the iris absorbs the red light.
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So the eye appears blue even though there's nothing blue in it.
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The denser those little pigment specks, the more brown it looks.
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Light-colored eyes like green, blue, and grey, aren't truly green, blue, or grey.
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They're just ... less and less brown.
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For most of the human population brown eyes dominate, but we find pockets of lighter-eyed
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people throughout the world.
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The mutation that originally caused blue eyes in European populations is at least 6,000
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years old, but it could be even older - maybe originating in Africa more than 10,000 years
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ago.
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Speaking of which, how is eye color inherited, anyway?
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Traits come, in part, from genes: little bits of DNA, passed down from parents.
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In grade school, maybe you heard that one gene determines eye color, and the brown-eyed
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trait always dominates over the blue-eyed trait.
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Or that blue-eyed parents can't have brown-eyed kids?
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Well that's all wrong.
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This kid, came from these parents.
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Eye color genetics is nowhere near as simple as we've been taught, and you're not secretly
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adopted… well, maybe you are, but you wouldn't know because of your eyes.
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Maybe you've heard of this monk, Gregor Mendel, who played with peas a lot back in
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the 1800s?
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Most our old explanations about eye color genetics came from his ideas, but more recently
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we've discovered that most traits don't fit neatly into Mendel's little squares
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because they involve several genes interacting together.
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Like, we have this gene.
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How strongly it's turned on determines how much pigment cells in our eyes make.
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What version of this gene you have is responsible for around 75 percent of having blue vs. brown
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eyes.
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BUT, another gene that lives close by can interfere with or even switch that gene off.
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So even if you have a “brown” trait here, you can end up with lighter eyes.
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And that's just two genes!
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In all, we've IDed at least 10 genes that influence eye color, and there are probably
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a lot more.
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Like height or intelligence, it's more like a genetic symphony, all playing with one another.
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No one's eyes can really be described by a single word, they fall on a continuous rainbow
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from brown to blue, and a lot in between.
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Turns out, your eye color is one of the most beautiful and unique things about you.
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What's an interesting way to describe YOUR eyes?
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Eye'll be looking for answers in the comments.
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Stay tuned for an even worse eye pun, but first…
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I want to tell you about the new PBS Digital Studios series Say It Loud, a celebration
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of Black history and culture, and its impact on how we live today.
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It's hosted by Evelyn from the Internets and Azie Dungey, they give you a hilarious
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take on identity and pop culture, and I recently joined them for an episode about DNA ancestry
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tests.
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Check out Say It Loud at the link in the description below.
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And finally, you know what they say about eye puns… could they BE any cornea?
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Stay curious!