Today's useless fact - How can someone have two different colored eyes?

According to the experts at "Scientific American", it was once believed that eye color was controlled by a single gene and inherited in a straightforward fashion (remember Mendel from high-school biology?). These days it's not quite that simple. We now believe that eye color is a polygenic trait.

Eye color is determined by the amount of melanin, a dark brown pigment, present in your irises. Blue eyes are due to a lack of melanin, while brown eyes indicate melanin- rich irises. Thus, people with darker hair and skin have higher levels of melanin and tend to have brown eyes, while people with lighter hair and skin have lower levels of melanin and usually have lighter colored eyes. This is also why many babies are born with blue eyes. Their eyes change color later as they begin to produce more melanin.

When an individual has different amounts of melanin in each of their irises, their eyes are different colors. Heterochromia iridium (the scientific name for two different color eyes in the same individual) is relatively rare in humans but common in some animals, such as horses, cats, and certain species of dogs. A variation on the condition is heterochromia iridis, in which an individual has a variety of colors within one iris.

Heterochromia iridium is thought to result from an alteration to one of the genes that controls eye color. This can be an inherited trait, although trauma and certain medications may result in increased or decreased pigmentation in one of the irises. Certain medical syndromes, such as Waardenburg syndrome, may also cause someone to have two different colored eyes.

Some people with this condition wear colored contact lenses so their irises match, while others take pride in their striking appearance. Check out the links in our Eye Color category for more.