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Q How does someone get different-colored eyes?

— James Lester, senior financial analyst at Morgridge Institute for Research

A Dr. Terri Young, chair of the University of Wisconsin-Madison department of ophthalmology and visual sciences and a pediatric ophthalmologist and researcher at the Wisconsin Institutes for Medical Research:

When the eye color, or iris color, is different between the two eyes, the condition is called iris heterochromia.

The iris is a very thin membrane in the eye with pigment in it. The amount of pigment determines whether you have light eyes, dark eyes or something in between. The iris has a black opening in the middle, the pupil, that allows light into the back of the eye.

There are multiple reasons you may have differences in color between the two eyes. In genetic mosaicism, it’s all about gene expression, or the appearance or characteristic that results from a gene. In this case, you have pigment genes expressing in one eye that are different from those genes expressed in the other eye.

You could have a genetic syndrome like Waardenburg syndrome or neurofibromatosis, both of which have implications beyond just eye color.

There’s also a situation where your iris looks like it’s a different color, but it’s only a small section or sliver of the iris.

When the eye forms, it starts off like a flat cookie sheet, which then wraps around and the edges meet. That seam, where the edges come together and the eye starts to cup and become more of a circle, may not close completely.

If it doesn’t close where the iris is, there can be a sliver of a different color or even an absence of the iris. The pupil looks more like a keyhole then, instead of a circle.

Blue Sky Science is a collaboration of the Wisconsin State Journal and the Morgridge Institute for Research.