Both humans and animals can have different colored eyes. Not many people know why this trait happens, though. Usually a genetic mutation called heterochromia causes the unique coloring. Sometimes the eyes will only vary slightly; other times the eyes will look completely different.
A rare abnormality, heterochromia can be caused by several different things, and most people are born with it. However, certain people develop the mutation later in life because of medical or chemical body changes. Unfortunately, heterochromia can also be an indicator of serious medical issues.
Shockingly, this trait is a lot more common than you might think.
You may have never heard of heterochromia, but you've probably seen someone with the condition. People with the unique mutation have irises that are partially or completely different in color. Certain individuals can have one blue eye and one brown one. Heterochromia is a Greek word, meaning "different colors," and the medical term is Heterochromia Iridis or Iridium, depending on the type.
This condition has affected humans for centuries. And it pops up in people from all different countries.
Heterochromia Is Closely Linked To Melanin Distribution
To better understand why heterochromia happens, one must first understand a little about eye color. The color in our eyes is actually a manifestation of pigment in the iris; the shade depends on melanin deposits. Brown eyes have a lot of melanin deposits, whereas blue eyes have a lack of melanin deposits.
There are two genes that control eye color. One on chromosome 15 codes for brown eyes, and one on chromosome 19 codes for green or blue eyes. While we don't know exactly how these two genes interact with each other, we do know that they dictate color deposits.
Once in a while, the concentration or distribution of melanin isn't uniform across the eyes, and heterochromia occurs. The eyes do not have a uniform level of melanin, so various parts of the eyes express opposing colors.
Not every case of heterochromia is the same. There are three types of the condition, each one looks very different from the others. In cases of complete heterochromia, people have totally different eyes. In cases of sectoral heterochromia, also known as partial heterochromia, one part of the iris is different from the rest. The whole eye isn't affected, though; parts of it may have different colored patches or spots.
Cases of central heterochromia are even more rare. Affected persons have two different colors in the same iris. This happens most often with lighter eyes, and rings of color will appear. The eye may also have spikes of color jutting out from the central ring of the iris. In these cases, the outer color is considered the true eye shade.
About One In A Hundred Americans Have The Condition
While this condition tends to occur somewhat regularly in cats and dogs, it's harder to notice in humans. In some cases, heterochromia exists without detection unless the eyes are subjected to certain light conditions. Sometimes, it's only visible in photographs. Other than eye color, there are no signs or symptoms of heterochromia.
About one in a hundred people in the United States have some form of the condition, though. You probably know at least one person with slightly mismatched eyes.
For the most part, heterochromia is transmitted genetically. Multi-colored eyes are actually dominant, so if one of your parents has the condition there's a 50 percent chance that you will too.
Interestingly, though, some people with the condition don't have parents with any signs of heterochromia.
Certain Genetic Alterations Can Cause Heterochromia
Heterochromia can also occur because of genetic mosaicism or chimerism. These genetic alterations happen when a person has more than one genetically-distinct population of cells. People with mosaicism have genetically different cells from a single zygote or fertilized egg. Basically, your body has genetic directions for two different, distinct versions of you. So you end up with different colors in each eye when that DNA difference occurs in genes affect eye color.
For people with chimerism, the method is very similar, except the genes come from more than one zygote or fertilized egg. In those cases, you end up with genetic directions for two totally different people. That's why the eyes differ.
