Hey smart pupils, Joe here. William Shakespeare called the eyes the “window
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 brown… ish. 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
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 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 colors… where 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
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… are 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. Only “earthy 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 particles… hey, 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 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? 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 “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 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 puns… could they BE any cornea? Stay curious!
