Eye color mocks easy rules
A blue iris. Photo courtesy of Corel Corporation.
Q: Why are blue eyes blue?
Some eyes are brown because of the brown pigment, melanin. An albino's eyes
are red because the lack of brown pigment lets the red color of red blood cells
show. But if brown eyes are brown because of the presence of melanin and red
eyes are red because of the lack of melanin, then why are blue eyes blue?
Mammals don't make a blue pigment. Steve B, Indiana
A: The colored part of the eye (the iris) regulates the amount of light that
the pupil lets in the eye. Thatís its job. Its color is a different story.
If the iris contains much brown pigment, then the iris reflects brown light
just like a brown shirt and appears brown. However, if the front layer of iris
cells (the stroma) contains little or no brown pigment, the tiny
loosely-organized stroma cells interact with blue light much more than with red
and lower-frequency light. The interaction causes the blue light to re-radiate
and scatter out the eye. An observer sees the out-going blue light and perceives
a blue iris.
Blue eyes, however, differ from red eyes in that they donít lack all brown
pigment. They have normal pigment in the back layer of iris cells (the iris
pigment epithelium [IPE]). Indeed, eyes of all colors have about the same amount
of pigment in the IPE, except an albinoís red eyes.
Blue eyes are blue for the same reason that the sky is blue. The stroma cells
function much like air molecules and tiny motes of dust in the atmosphere. These
particles are all small enough that the short-frequency light waves (i.e.,
violet and blue) are three times more likely to interact and scatter than red
Q: Seems that no one can figure this one out with a simple 2-gene square
chart. My mother has green eyes. My father has blue eyes. How did I get light
brown? Alex B.
A: Youíre right.
How do such eyes occur? Certainly the simple model we learned in school
about brown-eye color being dominant over blue falls short of an explanation.
Indeed that one-gene theory is kaput. There is no single gene for eye color.
Now, we know two major genes and other minor ones account for the tremendous
variation of human eye color, says
Richard A. Sturm,
a Principal Research Fellow at the Institute for Molecular Bioscience at the
University of Queensland in Brisbane, Australia and part of the team making this
discovery, which they reported in 2007.
The gene OCA2 produces a protein that allows the hair, skin and eyes to make
pigment (called melanin) that colors these body parts. The more pigment in the
eye, the darker it is. Much pigment results in brown eyes; little pigment
causes blue eyes.
A change that commonly occurs near the start of the OCA2 gene causes the eyes
to be either brown or blue. The gene change tells the pigment protein to
produce much pigment, which leads to brown eyes or to produce little, causing
blue eyes. It's an 'on' / 'off' order, "like switching on a light," says Sturm.
Another change, which also occurs commonly, happens to the pigment protein
under the control of the OCA2 gene. When the protein changes, its function
changes. It makes a different pigment that then colors the eyes green or
hazel. Sturm likens this process to "changing the light bulb from brown to
Q: When and why does a newbornís eye color change? Mamabaehr,
A: Babiesí eyes change from blue to their natural color by age three. In
three years, the eyes produce and store enough brown pigment to take on their
Before that, a babyís eyes lack melanin in the stromaómuch like blue-eyed
people. Special stroma cells (called melanocytes) make melanin as the baby ages.
We think that the amount and the distribution of melanin stored in these cells
account for iris color.
Q: What causes an individual to have one brown eye and one green eye?
A: Several things can cause this abnormality (called heterochromia irides):
faulty developmental pigment transport, local trauma either in the womb or
shortly after birth, or a benign genetic disorder. Other causes are
inflammation, freckle (diffuse nevus) of the iris, and Hornerís syndrome says
Joseph S. Elman, ophthalmology professor at Yale University.
color can change with age, WonderQuest
What exactly are hazel eyes,
and what color are they? WonderQuest
The eyes have it on multiple gene question by Rick Sturm, University of
Queensland, Australia, February 2007
The 2-gene model eye-color
Brannon, The Franklin Institute Online: Eye color genetics
MadSci Network: Genetics
John W. Kimballís biology pages: The human eye
--- add your comments to the discussion:
- When a person has two different colored eyes (one brown, one blue),
it can be caused by Waardenberg Syndrome, which runs in my family. I don't
know if I carry the gene or not. I do have eyes that changed color from very
dark brown as a young child to a light amber-brown as a teen then on to a pale
greenish-blue with yellow highlights and a thin band of light brown around the
pupil. People are constantly asking me what color my eyes are as the stare at
them, so they must be unusual. My dad's eyes were blue, my mom's are green. I
have siblings with blue, green and brown eyes. I got a little of each!
Susan, Spearfish, SD, USA