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!