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How Do Our Eyes Get Their Color?
Update: 2025-01-22
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There are no blue or green pigments in the human eye, so how do those eye colors occur? Learn about the complex genetics and light scattering that give our eyes their color (plus how rare different eye colors are) in this episode of BrainStuff, based on this article: https://health.howstuffworks.com/human-body/systems/eye/rarest-eye-colors.htm
See omnystudio.com/listener for privacy information.
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Transcript
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And we've got a new podcast, Full Circle.
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Every Wednesday, we're catching you up on what's going on in women's basketball.
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00:01:38
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00:02:06
Welcome to Brain Stuff, a production of iHeartRadio.
00:02:12
Hey, Brain Stuff, Lauren Volabom here.
00:02:18
Elizabeth Taylor was one of the most famous actors of the 20th century.
00:02:22
And one of her trademarks was her violet eyes.
00:02:26
Although it's difficult to see in photographs, a people who New Taylor claim they truly did have a purplish cast.
00:02:33
So how is that possible?
00:02:35
And what are the world's rarest eye colors?
00:02:39
The eye color is controlled by two factors-- the amount and distribution of pigments called melanins in your iris and the physical structure of your iris.
00:02:49
Let's break that down a little.
00:02:52
OK, the iris is the ring-shaped membrane that sits behind the eye's protective, clear cornea.
00:02:59
The purpose of the iris is to control the amount of light that enters your eye through the pupil, which is the hole in the center of the iris.
00:03:06
This happens with the motion of these layers of smooth and voluntary muscle in the iris that make it constrict or dilate, thus making your pupil smaller or bigger and letting less or more light into your eye.
00:03:20
The iris is made up of an intricate web of muscles and connective tissues.
00:03:25
The two main ones that contain melanins are the fibrous front layer, the stroma, and a thin back layer, the pigment epithelium.
00:03:34
Melanins are a group of pigments found in several places in our bodies, but relevant to today in our eyes, skin, and hair.
00:03:42
The two main types melanin found there are you melanin, which can be brown to black in color, and fiamalanin, which can be yellow to red.
00:03:52
The back layer of the iris can produce mostly you melanin.
00:03:55
The front layer can produce both types.
00:03:58
Brown eyes have a lot of these pigments.
00:04:00
Green and hazel eyes contain less, and blue eyes contain very little.
00:04:06
But those two are the only pigments in human eyes.
00:04:10
Like Elizabeth Taylor didn't have purple pigment in her irises.
00:04:14
If you have blue, green, or, yes, purple eyes, it's because you have a lack of melanins in different layers of your iris.
00:04:22
The amount and their distribution coupled with the way that light scatters through the layers results in eye color.
00:04:30
Researchers think that different eye colors might have evolved because mutations in melanin production proved useful in different environments.
00:04:39
For the article this episode is based on, How Stuff Works spoke with optimologist Isawoma Abogo M.D., whose name I hope I'm saying correctly, I did look it up.
00:04:48
She said, dark iris color is associated with less scattering of light in the eye.
00:04:54
This trait may be protective under conditions of bright sunlight and high ultraviolet radiation, alike for people who live in the equatorial regions of the world.
00:05:03
A blue eye color, on the other hand, is associated with greater light scattering in the eye.
00:05:08
In a higher level of melatonin suppression, traits that may have been adaptive under highly seasonal sunshine regimes in Northwestern, Eurasia.
00:05:18
How people end up with their own unique eye color is complex.
00:05:22
The research has discovered that at least 10 genes help determine eye color, though two genes located on chromosome 15 may influence it the most.
00:05:31
Abogo said, "A people used to believe eye color could be easily determined based on your parents' eyes, but the genetics of eye color is actually much more complicated."
00:05:40
Research has shown that the color of your eye may actually be linked genetically to the color of your skin and hair in some cases.
00:05:47
Basically, the color of your eyes determined by a complex mix of genes, some of which are still being studied.
00:05:55
Brown is the most common eye color in the world.
00:05:59
Between 70 and 80% of the world's population have eyes that are some shade of brown from tawny to nearly black.
00:06:06
These eyes all contain a lot of melanin, but the exact shade depends on how much the two layers of the iris contain and of what types.
00:06:16
Mostly, you melanin will create darker brown eyes, having more a pho melanin in the stroma creates lighter shades of brown.
00:06:24
It's thought that all ancient humans living more than 10,000 years ago had brown eyes.
00:06:29
The first light-eyed person probably had a genetic mutation that caused their body to produce less melanin, and this mutation was passed on to their descendants.
00:06:39
After brown, blue eyes are relatively common.
00:06:42
Between 8 and 10% of people in the world have them.
00:06:46
Again, there's no blue pigment in our eyes, though.
00:06:49
Rather, in blue-eyed people, there's very little melanin in either layer of the iris, and the stroma might contain no pigment at all.
00:06:58
But because the stroma is so textured and fibrous, light scatters through it and off of it, and the stroma appears blue.
00:07:07
This is called structural color.
00:07:10
It's similar to what gives butterfly wings and peacock feathers their colors.
00:07:15
But in the human eye, the stroma looks blue for essentially the same reason that the sky looks blue.
00:07:21
Because, although a full spectrum of light is hitting the stroma, a blue light scatters more than other visible wavelengths.
00:07:27
That means more blue light reaches the eye of the observer.
00:07:32
This is also why blue eyes can seem to change color.
00:07:35
The shade of blue that they appear changes based on how much light is available and how it scatters due to angles and other factors like the colors around you.
00:07:45
In dim light, for example, blue eyes can appear kind of stormy.
00:07:49
But in direct light, they can be very bright.
00:07:51
Because more light is reflecting off of the iris than happens with any other color of eye.
00:07:56
Historically, blue eyes and gray eyes have been combined into a single category.
00:08:03
But recently, researchers have discovered that there are some differences.
00:08:07
Around 3% of the world's population have gray eyes.
00:08:11
And like most light colored eyes, the coloration is the product of very little melanin in the iris.
00:08:17
As with blue eyes, the stroma may have no pigment at all.
00:08:20
However, gray eyes have more collagen in the stroma than blue eyes, affecting the way that light scatters within the iris.
00:08:27
But okay, let's talk about green eyes.
00:08:32
Only about 2% of the world's population sports is eye color.
00:08:36
And green eyes are far more common in parts of Europe than in the world at large, and women have them more often than men.
00:08:43
For green eyed people, the back layer of the iris has a low concentration of eumelin.
00:08:48
And the stroma contains a low amount of fail melanin.
00:08:52
So the green color you see is a mixture of the different pigments and a bit of the light scattering that you see in blue eyes.
00:09:01
There's not much data on hazel eyes, which are mixed green brown, but it's thought that around 5% of the world's population has them.
00:09:09
This coloration is probably a result of different concentrations of melanin in different sections of the iris, and a mix of eumelin and fail melanin in the stroma.
00:09:20
But one of the rarest eye colors in the world isn't just one color, it's two.
00:09:26
People with a condition called heterochromia have irises of two different colors.
00:09:30
Less than 1% of the world's population has this.
00:09:34
In complete heterochromia, the eyes have two completely different colored irises.
00:09:38
In partial heterochromia, just a portion of the iris is a different color from the rest.
00:09:44
An infant can be born with heterochromia and have completely healthy eyes, but it can also be acquired later as a symptom of an injury, disease, or syndrome.
00:09:54
About equally as rare are red or violet eyes, and they often point to an underlying condition-- albinism, a genetic condition in which a person is born with little or no melanin in their entire body,
00:10:07
albinism affects the appearance of hair, skin, and eyes.
00:10:11
The eyes can appear a very pale blue, a very pale purple, or even reddish in some lights, as the result of light reflecting off of blood vessels in the eye.
00:10:21
When just a very little bit of melanin is present, these red reflections mix with the pigment to create violet eyes.
00:10:29
However, Elizabeth Taylor doesn't seem to have had albinism.
00:10:33
It's more likely that she had a variant of blue eyes that appeared particularly rich due to their particular structure, and maybe she played their color up with complementary colors in her makeup,
00:10:44
clothing, and hair.
00:10:46
Again, the genetics and physics of how our eyes appear are complicated and still being studied.
00:10:52
Today's episode is based on the article, Do You Have One of These Six Rareest Eye Colors in the World on hasduffworks.com,
00:11:03
written by Jess Lynn Shields?
00:11:05
Brain stuff is production of iHeartRadio in partnership with hasduffworks.com and is produced by Tyler Cling.
00:11:10
For more podcasts, my heart radio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows.
00:11:17
[MUSIC]
00:11:29
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00:11:33
With up first from MPR, though, it doesn't have to be.
00:11:35
Welcome to 15 easy minutes of breaking news, clarity on international and national affairs, and a casual tone that you can take in with breakfast.
00:11:43
Begin your day, informed, ready, and refreshed.
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Begin your day with up first.
00:11:48
Subscribe to Up First from NPR on the iHeartRadio app, or wherever you get your podcasts.
00:11:53
[MUSIC]
00:11:58
John Stewart is back at the Daily Show and he's bringing his signature wit and insight straight to your ears, with the Daily Show Ears Edition podcast.
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Dive into John's unique take on the biggest topics in politics, entertainment, sports, and more.
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Join by the sharp voices of the shows, correspondence, and contributors.
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And with extended interviews and exclusive weekly headline roundups, this podcast gives you content you won't find anywhere else.
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00:12:26
Listen on the iHeartRadio app, Apple Podcasts, or wherever you get your podcasts.
00:12:31
[MUSIC]
00:12:36
I'm so sick of hearing men talk about women's basketball.
00:12:39
This is Lexi Brown and Mariah Rose, and we've got a new podcast full circle.
00:12:44
Everyone's day we're catching you up on what's going on in women's basketball.
00:12:48
We've got you with analysis, insight stories, and a little bit of tea.
00:12:52
Full circle is an iHeart Women's Sports production and partnership with Deep Blue Sports and Entertainment.
00:12:56
Listen to full circle on the iHeartRadio app, Apple Podcasts, or wherever you get your podcasts.
00:13:01
Presented by Elf Beauty, founding partner of iHeart Women's Sports.
00:13:06
The forces shaping markets and the economy are often hiding behind a blur of numbers.
00:13:10
So that's why we created the big take from Bloomberg podcasts.
00:13:14
To give you the context you need to make sense of it all.
00:13:17
Every day in just 15 minutes, we dive into one global business story that matters.
00:13:22
You'll hear from Bloomberg journalists, like Matt Levine.
00:13:25
A lot of this boomstock stuff is I think embarrassing to the SEC.
00:13:28
Follow the big take podcast on the iHeartRadio app, Apple Podcasts, or wherever you listen.
00:13:35