Heterochromia Iridium: The Case of the Different Colored Eyes

The back cover of Graceling.

In the fantasy world of Graceling, certain people are “graced” with extraordinary powers; some, like fighting, cooking and mind-reading (Edward Cullen style) are useful, while others, such as holding your breath or climbing a tree, are useless. These “gracelings” may receive their powers at a day old or at 10 years during their childhood. Parents watch the eyes of their children warily until they are grown, because the mark of a graceling is two eyes of different colors. When gracelings are marked, they are immediately sent to the king until they learn what their grace is. If it is something useful, they stay at court, but if not, they return home, but to no advantage, because gracelings are avoided and considered “freaks of nature.” In this story, both main characters are gracelings. Katsa has one green and one blue eye, while Po has one silver and one gold eye.  Ever since reading The Clique series, when Cam Fisher, Claire’s boyfriend, has one blue and one green eye, I have been interested in the idea, and I decided to take a closer look into how people can have different color eyes.

Famous American actress, Kate
Bosworth, has one hazel
eye and one blue eye. You
might recognize her from
Remember the Titans!!

            People used to believe that only one gene controlled eye color, and that it could be inherited straightforward, like in Mendelian genetics (like the Punnet squares we learned about in biology!!) However, now people believe that eye color is a polygenic trait, meaning that many genes influence one eye’s color. The main thing that affects eye color is melanin. The more melanin an iris contains, the darker it becomes. That’s why people who produce more melanin have dark skin and brown eyes while people who produce little melanin have light skin and blue eyes.

 Odd-eyed, Turkish Angoran cats like this one
are considered "national treasures" in Turkey.

            

While heterochromia iridium (the fancy scientific definition for one individual having two different colored eyes) is common in dogs, cats, and horses, it is fairly uncommon in humans, but not impossible. It is caused when each iris contains a different amount of melanin, resulting in two eyes of different colors. This difference is melanin is believed to be caused by an altered gene that affects eye color. This trait can be inherited, caused by injury or trauma near time of birth or later, or caused by medical problems like Waardenberg syndrome.

 ~I think it would be so cool to have different colored eyes, don’t you? They are so special and interesting to look at. At least I have my birthmark…~

For my research on this blog, I used this website as my main starting point because it was written by an expert, but it was a little hard to follow, so for all of you non-scientists out there like me, use this other website if you want to learn more, because they used the same source, but explained better.