Hey smart pupils, Joe here.

William Shakespeare called the eyes the window to the soul.

Did he actually say that?

But anyway, 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 brown-ish with a sort of hazel ring going on.

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

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 out because 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 stared at someone's eyeball from six inches away?

It's kind of weird.

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

But where do these eye colors come from?

Well, don't blink.

Here comes the science.

[MUSIC PLAYING] 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.

Well, that little colored bit on the front is called the iris.

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 browns, they're all a trick of physics.

If you pulled out your irises and tried to look at the pigment molecules inside, well, 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, only earthy tones.

In every color of eye, the back-most inside layer 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 layer of your iris are full of pigment.

If you have blue or blue-gray, 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 of 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 wavelength 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 particles, but hey, this is science-- details matter.

So how is this happening inside the eye?

Well, 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 gray, well, they aren't truly green, blue, or gray.

They're just less and less brown.

For most of the human population, brown eyes dominate.

But we do 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.

Now in grade school, maybe you heard that one gene determines eye color and that the brown-eyed trait always dominates over the blue-eyed trait, or maybe 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 adopted.

Well, 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.

Well, most of 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 about 75% of having blue versus brown eyes.

But another gene that lives close by can interfere with or even switch that gene off.

So even if you have a brown trait 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 in 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.

And finally, you know what they say about eye puns-- could they be any cornea?

[RIMSHOT] Stay curious.

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