Having Two Different Colored Eyes
Heterochromia is Caused by Genetics, Injury and/or Disease
Heterochromia or two different colored eyes can happen in people because of lost or damaged melanocytes and/or genetic conditions like chimerism or mosaicism.
What do David Bowie, Jane Seymour, Dan Ackroyd, and Christopher Walken have in common? They all have heterochromia or two different colored eyes.
There are many ways to end up with heterochromia. An understanding of how eye color works is necessary before understanding how to end up with two different colored eyes.
The Nuts and Bolts of Eye Color
Eye color is the result of how much pigment is made in the front part of the eye. Little or no pigment gives blue eyes, some pigment results in green, and lots of pigment gives brown eyes. The amount of pigment is determined by at least two genes in special cells called melanocytes.
A common form of heterochromia is one blue eye and one different colored eye. One way to end up with a blue eye is if part of one eye is missing melanocytes. Another way is if an eye color gene only “works” in one eye.
Lost Melanocytes
Getting color in hair, skin, or eyes is surprisingly complex. Once a melanocyte develops, it often needs to migrate to its proper place. Sometimes melanocytes get lost on the way to where they should end up.
When a melanocyte doesn’t make it to the right place, that spot will not have color. If it is skin or hair, it will be a white spot. If it is the eye, then the eye will be blue.
Many factors can affect a melanocyte’s well being. For example, an injury during development can cause them to die off. And there are certain diseases that can cause a melanocyte not to develop or migrate to the right spot. The most common is called Waardenburg syndrome.
People with Waardenburg syndrome sometimes having a shock of white hair or two different colored eyes. It is also why some dog breeds can sometimes have two different colored eyes.
Besides lost melanocytes, a less common way to end up with two different colored eyes is when an eye color gene works in one eye but not the other. Two ways that this can happen are called mosaicism and chimerism.
Chimeras and Mosaics
Both chimeras and mosaics have some cells in their body with one set of DNA and other cells with a different set of DNA. In chimeras, the DNAs are as different as any two siblings’ DNA (which makes sense since chimeras are fused fraternal twins). The DNA difference in mosaics is much smaller. Often the difference can only be at a single gene.
A chimera can have two different colored eyes just like any two siblings can—because each cell has different eye color genes. A mosaic can have two different colored eyes if the DNA difference happens to be in an eye color gene.
There are many other possible reasons for having two different colored eyes. For example, David Bowie's different eye colors comes from an injury that caused one pupil to be permanently dilated. Another idea about how this can happen is if an early viral infection while in the womb turns an eye color gene on or off in just one eye. Occasionally it can be a sign of a serious disease.
As scientists learn more and more about how eye color works, they’ll be able to figure out more about why some people have two different colored eyes.
