January 22, 2025 • 9 mins
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
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Episode Transcript
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Speaker 1 (00:01):
Welcome to Brainstuff, a production of iHeartRadio. Hey Brainstuff, Lauren
Vogelbaum here. Elizabeth Taylor was one of the most famous
actors of the twentieth century, and one of her trademarks
was her violet eyes. Although it's difficult to see in
photographs a people who knew Taylor claim they truly did
(00:23):
have a purplish cast. So how is that possible? And
what are the world's rarest eye colors? 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. Let's break that down a little, okay.
(00:44):
The iris is the ring shaped membrane that sits behind
the eyes protective clear cornea. 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. This happens with the motion of
these layers of smooth, involuntary muscle in the iris that
make it constrict or dilate, thus making your pupil smaller
(01:07):
or bigger and letting less or more light into your eye.
The iris is made up of an intricate web of
muscles and connective tissues. The two main ones that contain
melanins are the fibrous front layer, the stroma, and a
thin back layer, the pigment epithelium. Melanins are a group
(01:27):
of pigments found in several places in our bodies, but
relevant to today in our eyes, skin, and hair. The
two main types melanin found there are eumelanin, which can
be brown to black in color, and fiamelanin, which can
be yellow to red. The back layer of the iris
can produce mostly eumelanin, the front layer can produce both types.
(01:49):
Brown eyes have a lot of these pigments, Green and
hazel eyes contain less, and blue eyes contain very little,
but those two are the only pigments, and human eyes,
like Elizabeth Taylor, didn't have purple pigment in her irises.
If you have blue, green, or yiss purple eyes, it's
(02:10):
because you have a lack of melanins in different layers
of your iris. The amount and their distribution, coupled with
the way that light scatters through the layers, results in
eye color. Researchers think that different eye colors might have
evolved because mutations and melanin production proved useful in different environments.
(02:31):
For the article, this episode is based on How Stuff Works.
Spoke with optimologist Osuwoma Abogo, MD, whose name I hope
I'm saying correctly. I did look it up. She said,
dark iris color is associated with less scattering of light
in the eye. This trait may be protective under conditions
of bright sunlight and high ultraviolet radiation, alike for people
(02:52):
who live in the equatorial regions of the world. Blue
eye color, on the other hand, is associated with greater
light scattering in the eye a higher level of melatonin suppression,
traits that may have been adaptive under highly seasonal sunshine
regimes in northwestern Eurasia. How people end up with their
own unique eye color is complex. The research has discovered
(03:15):
that at least ten genes help determine eye color, though
two genes located on chromosome fifteen may influence it the most.
Abuga 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. Research
has shown that the color of your eye may actually
be linked genetically to the color of your skin and
(03:37):
hair in some cases. Basically, the color of your eyes
determined by a complex mix of genes, some of which
are still being studied. Brown is the most common eye
color in the world. Between seventy and eighty percent of
the world's population have eyes that are some shade of brown,
from tawny to nearly black. These eyes all contain a
(03:59):
lot of melanin, but the exact shade depends on how
much the two layers of the iris contain and of
what types. Mostly eumelanin will create darker brown eyes, having
more feomelanin in the stroma creates lighter shades of brown.
It's thought that all ancient humans living more than ten
thousand years ago had brown eyes. The first light eyed
(04:22):
person probably had a genetic mutation that caused their body
to produce less melanin, and this mutation was passed on
to their descendants after brown, blue eyes are relatively common,
between eight and ten percent of people in the world
have them. Again, there's no blue pigment in our eyes, though,
or rather in blue eyed people. There's very little melanin
(04:44):
in either layer of the iris, and the stroma might
contain no pigment at all, but because the stroma is
so textured and fibrous, light scatters through it and off
of it, and the stroma appears blue. This is called
structural color. It's similar to what gives butterfly wings and
(05:04):
peacock feathers their colors. But in the human eye, the
strama looks blue for essentially the same reason that the
sky looks blue, Because although a full spectrum of light
is hitting the strama a, blue light scatters more than
other visible wavelengths. That means more blue light reaches the
eye of the observer. This is also why blue eyes
(05:25):
can seem to change color. 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. In dim light, for example,
blue eyes can appear kind of stormy, but in direct
light they can be very bright because more light is
reflecting off of the iris than happens with any other
(05:47):
color of eye. Historically, blue eyes and gray eyes have
been combined into a single category, but recently researchers have
discovered that there are some differences. Around three eighty percent
of the world's population have gray eyes and Like most
light colored eyes, the coloration is the product of very
little melanin in the iris. As with blue eyes, the
(06:09):
stroma may have no pigment at all. However, gray eyes
have more collagen in the stroma than blue eyes, affecting
the way that light scatters within the iris. But okay,
let's talk about green eyes. Only about two percent of
the world's population sports this eye color. Green eyes are
far more common in parts of Europe than in the
(06:30):
world at large, and women have them more often than men.
For green eyed people, the back layer of the iris
has a low concentration of U melanin and the stroma
contains a low amount of faeomelanin, 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. There's not much data on hazel eyes, which
(06:55):
are mixed green brown, but it's thought that around five
percent of the world's population has them. This coloration is
probably a result of different concentrations of melanin in different
sections of the iris, and a mix of few melanin
and fao melanin in the stroma. But one of the
rarest eye colors in the world isn't just one color,
(07:16):
it's two. People with a condition called heterochromia have irises
of two different colors. Less than one percent of the
world's population has this. In complete heterochromia, the eyes have
two completely different colored irises. In partial heterochromia, just a
portion of the iris is a different color from the rest.
An infant can be born with heterochromia and have completely
(07:38):
healthy eyes, but it can also be acquired later as
a symptom of an injury, disease, or syndrome. 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. Albinism the appearance of hair, skin,
(08:02):
and eyes. The eyes can a pair 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. When just a very little bit of
melanin is present, these red reflections mix with the pigment
to create violet eyes. However, Elizabeth Taylor doesn't seem to
(08:24):
have had albinism. 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 complimentary colors in her makeup, clothing, and hair. Again,
the genetics and physics of how our eyes appear are
complicated and still being studied. Today's episode is based on
(08:50):
the article do you have one of these six rarest
eye colors in the world? On how stuffworks dot com
written by Jeslyn Shields. Brainstuff is production of iHeart Radio
in partnership with how stuffworkst Com and is produced by
Tyler Klang. Four more podcasts my heart Radio, visit the
iHeartRadio app, Apple Podcasts, or wherever you listen to your
favorite shows.
Welcome to Brainstuff, a production of iHeartRadio. Hey Brainstuff, Lauren
Vogelbaum here. Elizabeth Taylor was one of the most famous
actors of the twentieth century, and one of her trademarks
was her violet eyes. Although it's difficult to see in
photographs a people who knew Taylor claim they truly did
(00:23):
have a purplish cast. So how is that possible? And
what are the world's rarest eye colors? 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. Let's break that down a little, okay.
(00:44):
The iris is the ring shaped membrane that sits behind
the eyes protective clear cornea. 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. This happens with the motion of
these layers of smooth, involuntary muscle in the iris that
make it constrict or dilate, thus making your pupil smaller
(01:07):
or bigger and letting less or more light into your eye.
The iris is made up of an intricate web of
muscles and connective tissues. The two main ones that contain
melanins are the fibrous front layer, the stroma, and a
thin back layer, the pigment epithelium. Melanins are a group
(01:27):
of pigments found in several places in our bodies, but
relevant to today in our eyes, skin, and hair. The
two main types melanin found there are eumelanin, which can
be brown to black in color, and fiamelanin, which can
be yellow to red. The back layer of the iris
can produce mostly eumelanin, the front layer can produce both types.
(01:49):
Brown eyes have a lot of these pigments, Green and
hazel eyes contain less, and blue eyes contain very little,
but those two are the only pigments, and human eyes,
like Elizabeth Taylor, didn't have purple pigment in her irises.
If you have blue, green, or yiss purple eyes, it's
(02:10):
because you have a lack of melanins in different layers
of your iris. The amount and their distribution, coupled with
the way that light scatters through the layers, results in
eye color. Researchers think that different eye colors might have
evolved because mutations and melanin production proved useful in different environments.
(02:31):
For the article, this episode is based on How Stuff Works.
Spoke with optimologist Osuwoma Abogo, MD, whose name I hope
I'm saying correctly. I did look it up. She said,
dark iris color is associated with less scattering of light
in the eye. This trait may be protective under conditions
of bright sunlight and high ultraviolet radiation, alike for people
(02:52):
who live in the equatorial regions of the world. Blue
eye color, on the other hand, is associated with greater
light scattering in the eye a higher level of melatonin suppression,
traits that may have been adaptive under highly seasonal sunshine
regimes in northwestern Eurasia. How people end up with their
own unique eye color is complex. The research has discovered
(03:15):
that at least ten genes help determine eye color, though
two genes located on chromosome fifteen may influence it the most.
Abuga 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. Research
has shown that the color of your eye may actually
be linked genetically to the color of your skin and
(03:37):
hair in some cases. Basically, the color of your eyes
determined by a complex mix of genes, some of which
are still being studied. Brown is the most common eye
color in the world. Between seventy and eighty percent of
the world's population have eyes that are some shade of brown,
from tawny to nearly black. These eyes all contain a
(03:59):
lot of melanin, but the exact shade depends on how
much the two layers of the iris contain and of
what types. Mostly eumelanin will create darker brown eyes, having
more feomelanin in the stroma creates lighter shades of brown.
It's thought that all ancient humans living more than ten
thousand years ago had brown eyes. The first light eyed
(04:22):
person probably had a genetic mutation that caused their body
to produce less melanin, and this mutation was passed on
to their descendants after brown, blue eyes are relatively common,
between eight and ten percent of people in the world
have them. Again, there's no blue pigment in our eyes, though,
or rather in blue eyed people. There's very little melanin
(04:44):
in either layer of the iris, and the stroma might
contain no pigment at all, but because the stroma is
so textured and fibrous, light scatters through it and off
of it, and the stroma appears blue. This is called
structural color. It's similar to what gives butterfly wings and
(05:04):
peacock feathers their colors. But in the human eye, the
strama looks blue for essentially the same reason that the
sky looks blue, Because although a full spectrum of light
is hitting the strama a, blue light scatters more than
other visible wavelengths. That means more blue light reaches the
eye of the observer. This is also why blue eyes
(05:25):
can seem to change color. 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. In dim light, for example,
blue eyes can appear kind of stormy, but in direct
light they can be very bright because more light is
reflecting off of the iris than happens with any other
(05:47):
color of eye. Historically, blue eyes and gray eyes have
been combined into a single category, but recently researchers have
discovered that there are some differences. Around three eighty percent
of the world's population have gray eyes and Like most
light colored eyes, the coloration is the product of very
little melanin in the iris. As with blue eyes, the
(06:09):
stroma may have no pigment at all. However, gray eyes
have more collagen in the stroma than blue eyes, affecting
the way that light scatters within the iris. But okay,
let's talk about green eyes. Only about two percent of
the world's population sports this eye color. Green eyes are
far more common in parts of Europe than in the
(06:30):
world at large, and women have them more often than men.
For green eyed people, the back layer of the iris
has a low concentration of U melanin and the stroma
contains a low amount of faeomelanin, 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. There's not much data on hazel eyes, which
(06:55):
are mixed green brown, but it's thought that around five
percent of the world's population has them. This coloration is
probably a result of different concentrations of melanin in different
sections of the iris, and a mix of few melanin
and fao melanin in the stroma. But one of the
rarest eye colors in the world isn't just one color,
(07:16):
it's two. People with a condition called heterochromia have irises
of two different colors. Less than one percent of the
world's population has this. In complete heterochromia, the eyes have
two completely different colored irises. In partial heterochromia, just a
portion of the iris is a different color from the rest.
An infant can be born with heterochromia and have completely
(07:38):
healthy eyes, but it can also be acquired later as
a symptom of an injury, disease, or syndrome. 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. Albinism the appearance of hair, skin,
(08:02):
and eyes. The eyes can a pair 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. When just a very little bit of
melanin is present, these red reflections mix with the pigment
to create violet eyes. However, Elizabeth Taylor doesn't seem to
(08:24):
have had albinism. 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 complimentary colors in her makeup, clothing, and hair. Again,
the genetics and physics of how our eyes appear are
complicated and still being studied. Today's episode is based on
(08:50):
the article do you have one of these six rarest
eye colors in the world? On how stuffworks dot com
written by Jeslyn Shields. Brainstuff is production of iHeart Radio
in partnership with how stuffworkst Com and is produced by
Tyler Klang. Four more podcasts my heart Radio, visit the
iHeartRadio app, Apple Podcasts, or wherever you listen to your
favorite shows.
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