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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. That’s 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 don’t 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 albino’s 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 light.

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: You’re 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 green."

Q: When and why does a newborn’s eye color change?  Mamabaehr, Philadelphia, Pennsylvania

A: Babies’ 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 natural shade.

Before that, a baby’s eyes lack melanin in the stroma—much 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? Mary Eileen

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 Horner’s syndrome says Joseph S. Elman, ophthalmology professor at Yale University.

Further Surfing:

Eye 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 calculator. 

JJ Brannon, The Franklin Institute Online: Eye color genetics

MadSci Network: Genetics

John W. Kimball’s biology pages: The human eye

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