WEBVTT - How Do Our Eyes Get Their Color?

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<v Speaker 1>Welcome to Brainstuff, a production of iHeartRadio. Hey Brainstuff, Lauren

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<v Speaker 1>Vogelbaum here. Elizabeth Taylor was one of the most famous

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<v Speaker 1>actors of the twentieth century, and one of her trademarks

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<v Speaker 1>was her violet eyes. Although it's difficult to see in

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<v Speaker 1>photographs a people who knew Taylor claim they truly did

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<v Speaker 1>have a purplish cast. So how is that possible? And

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<v Speaker 1>what are the world's rarest eye colors? The eye color

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<v Speaker 1>is controlled by two factors, the amount and distribution of

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<v Speaker 1>pigments called melanins in your iris, and the physical structure

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<v Speaker 1>of your iris. Let's break that down a little, okay.

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<v Speaker 1>The iris is the ring shaped membrane that sits behind

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<v Speaker 1>the eyes protective clear cornea. The purpose of the iris

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<v Speaker 1>is to control the amount of light that enters your

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<v Speaker 1>eye through the pupil, which is the hole in the

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<v Speaker 1>center of the iris. This happens with the motion of

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<v Speaker 1>these layers of smooth, involuntary muscle in the iris that

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<v Speaker 1>make it constrict or dilate, thus making your pupil smaller

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<v Speaker 1>or bigger and letting less or more light into your eye.

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<v Speaker 1>The iris is made up of an intricate web of

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<v Speaker 1>muscles and connective tissues. The two main ones that contain

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<v Speaker 1>melanins are the fibrous front layer, the stroma, and a

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<v Speaker 1>thin back layer, the pigment epithelium. Melanins are a group

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<v Speaker 1>of pigments found in several places in our bodies, but

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<v Speaker 1>relevant to today in our eyes, skin, and hair. The

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<v Speaker 1>two main types melanin found there are eumelanin, which can

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<v Speaker 1>be brown to black in color, and fiamelanin, which can

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<v Speaker 1>be yellow to red. The back layer of the iris

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<v Speaker 1>can produce mostly eumelanin, the front layer can produce both types.

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<v Speaker 1>Brown eyes have a lot of these pigments, Green and

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<v Speaker 1>hazel eyes contain less, and blue eyes contain very little,

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<v Speaker 1>but those two are the only pigments, and human eyes,

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<v Speaker 1>like Elizabeth Taylor, didn't have purple pigment in her irises.

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<v Speaker 1>If you have blue, green, or yiss purple eyes, it's

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<v Speaker 1>because you have a lack of melanins in different layers

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<v Speaker 1>of your iris. The amount and their distribution, coupled with

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<v Speaker 1>the way that light scatters through the layers, results in

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<v Speaker 1>eye color. Researchers think that different eye colors might have

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<v Speaker 1>evolved because mutations and melanin production proved useful in different environments.

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<v Speaker 1>For the article, this episode is based on How Stuff Works.

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<v Speaker 1>Spoke with optimologist Osuwoma Abogo, MD, whose name I hope

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<v Speaker 1>I'm saying correctly. I did look it up. She said,

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<v Speaker 1>dark iris color is associated with less scattering of light

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<v Speaker 1>in the eye. This trait may be protective under conditions

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<v Speaker 1>of bright sunlight and high ultraviolet radiation, alike for people

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<v Speaker 1>who live in the equatorial regions of the world. Blue

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<v Speaker 1>eye color, on the other hand, is associated with greater

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<v Speaker 1>light scattering in the eye a higher level of melatonin suppression,

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<v Speaker 1>traits that may have been adaptive under highly seasonal sunshine

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<v Speaker 1>regimes in northwestern Eurasia. How people end up with their

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<v Speaker 1>own unique eye color is complex. The research has discovered

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<v Speaker 1>that at least ten genes help determine eye color, though

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<v Speaker 1>two genes located on chromosome fifteen may influence it the most.

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<v Speaker 1>Abuga said, A people used to believe eye color could

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<v Speaker 1>be easily determined based on your parents' eyes, but the

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<v Speaker 1>genetics of eye color is actually much more complicated. Research

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<v Speaker 1>has shown that the color of your eye may actually

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<v Speaker 1>be linked genetically to the color of your skin and

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<v Speaker 1>hair in some cases. Basically, the color of your eyes

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<v Speaker 1>determined by a complex mix of genes, some of which

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<v Speaker 1>are still being studied. Brown is the most common eye

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<v Speaker 1>color in the world. Between seventy and eighty percent of

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<v Speaker 1>the world's population have eyes that are some shade of brown,

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<v Speaker 1>from tawny to nearly black. These eyes all contain a

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<v Speaker 1>lot of melanin, but the exact shade depends on how

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<v Speaker 1>much the two layers of the iris contain and of

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<v Speaker 1>what types. Mostly eumelanin will create darker brown eyes, having

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<v Speaker 1>more feomelanin in the stroma creates lighter shades of brown.

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<v Speaker 1>It's thought that all ancient humans living more than ten

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<v Speaker 1>thousand years ago had brown eyes. The first light eyed

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<v Speaker 1>person probably had a genetic mutation that caused their body

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<v Speaker 1>to produce less melanin, and this mutation was passed on

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<v Speaker 1>to their descendants after brown, blue eyes are relatively common,

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<v Speaker 1>between eight and ten percent of people in the world

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<v Speaker 1>have them. Again, there's no blue pigment in our eyes, though,

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<v Speaker 1>or rather in blue eyed people. There's very little melanin

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<v Speaker 1>in either layer of the iris, and the stroma might

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<v Speaker 1>contain no pigment at all, but because the stroma is

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<v Speaker 1>so textured and fibrous, light scatters through it and off

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<v Speaker 1>of it, and the stroma appears blue. This is called

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<v Speaker 1>structural color. It's similar to what gives butterfly wings and

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<v Speaker 1>peacock feathers their colors. But in the human eye, the

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<v Speaker 1>strama looks blue for essentially the same reason that the

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<v Speaker 1>sky looks blue, Because although a full spectrum of light

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<v Speaker 1>is hitting the strama a, blue light scatters more than

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<v Speaker 1>other visible wavelengths. That means more blue light reaches the

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<v Speaker 1>eye of the observer. This is also why blue eyes

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<v Speaker 1>can seem to change color. The shade of blue that

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<v Speaker 1>they appear changes based on how much light is available

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<v Speaker 1>and how it scatters due to angles and other factors

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<v Speaker 1>like the colors around you. In dim light, for example,

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<v Speaker 1>blue eyes can appear kind of stormy, but in direct

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<v Speaker 1>light they can be very bright because more light is

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<v Speaker 1>reflecting off of the iris than happens with any other

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<v Speaker 1>color of eye. Historically, blue eyes and gray eyes have

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<v Speaker 1>been combined into a single category, but recently researchers have

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<v Speaker 1>discovered that there are some differences. Around three eighty percent

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<v Speaker 1>of the world's population have gray eyes and Like most

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<v Speaker 1>light colored eyes, the coloration is the product of very

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<v Speaker 1>little melanin in the iris. As with blue eyes, the

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<v Speaker 1>stroma may have no pigment at all. However, gray eyes

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<v Speaker 1>have more collagen in the stroma than blue eyes, affecting

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<v Speaker 1>the way that light scatters within the iris. But okay,

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<v Speaker 1>let's talk about green eyes. Only about two percent of

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<v Speaker 1>the world's population sports this eye color. Green eyes are

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<v Speaker 1>far more common in parts of Europe than in the

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<v Speaker 1>world at large, and women have them more often than men.

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<v Speaker 1>For green eyed people, the back layer of the iris

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<v Speaker 1>has a low concentration of U melanin and the stroma

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<v Speaker 1>contains a low amount of faeomelanin, So the green color

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<v Speaker 1>you see is a mixture of the different pigments and

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<v Speaker 1>a bit of the light scattering that you see in

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<v Speaker 1>blue eyes. There's not much data on hazel eyes, which

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<v Speaker 1>are mixed green brown, but it's thought that around five

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<v Speaker 1>percent of the world's population has them. This coloration is

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<v Speaker 1>probably a result of different concentrations of melanin in different

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<v Speaker 1>sections of the iris, and a mix of few melanin

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<v Speaker 1>and fao melanin in the stroma. But one of the

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<v Speaker 1>rarest eye colors in the world isn't just one color,

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<v Speaker 1>it's two. People with a condition called heterochromia have irises

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<v Speaker 1>of two different colors. Less than one percent of the

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<v Speaker 1>world's population has this. In complete heterochromia, the eyes have

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<v Speaker 1>two completely different colored irises. In partial heterochromia, just a

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<v Speaker 1>portion of the iris is a different color from the rest.

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<v Speaker 1>An infant can be born with heterochromia and have completely

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<v Speaker 1>healthy eyes, but it can also be acquired later as

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<v Speaker 1>a symptom of an injury, disease, or syndrome. About equally

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<v Speaker 1>as rare are red or violet eyes, and they often

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<v Speaker 1>point to an underlying condition. Albinism, a genetic condition in

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<v Speaker 1>which a person is born with little or no melanin

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<v Speaker 1>in their entire body. Albinism the appearance of hair, skin,

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<v Speaker 1>and eyes. The eyes can a pair a very pale blue,

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<v Speaker 1>a very pale purple, or even reddish in some lights

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<v Speaker 1>as the result of light reflecting off of blood vessels

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<v Speaker 1>in the eye. When just a very little bit of

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<v Speaker 1>melanin is present, these red reflections mix with the pigment

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<v Speaker 1>to create violet eyes. However, Elizabeth Taylor doesn't seem to

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<v Speaker 1>have had albinism. It's more likely that she had a

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<v Speaker 1>variant of blue eyes that appeared particularly rich due to

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<v Speaker 1>their particular structure, and maybe she played their color up

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<v Speaker 1>with complimentary colors in her makeup, clothing, and hair. Again,

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<v Speaker 1>the genetics and physics of how our eyes appear are

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<v Speaker 1>complicated and still being studied. Today's episode is based on

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<v Speaker 1>the article do you have one of these six rarest

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<v Speaker 1>eye colors in the world? On how stuffworks dot com

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<v Speaker 1>written by Jeslyn Shields. Brainstuff is production of iHeart Radio

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<v Speaker 1>in partnership with how stuffworkst Com and is produced by

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<v Speaker 1>Tyler Klang. Four more podcasts my heart Radio, visit the

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<v Speaker 1>iHeartRadio app, Apple Podcasts, or wherever you listen to your

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