Re: Why do some people have two different coloured eyes?

Hi Agnes,

thanks for your question. Your hypothesis that eye color is determined by melanin levels is correct. Genetically speaking, eye color is a polygenic trait, meaning that multiple genes are responsible for iris pigmentation. Two genes that specifically code for eye color in humans have been identified so far, and more probably exist. The condition you describe of having two differently colored eyes is called heterochromia iridium. It is pretty rare in humans, and there are some human diseases for which this is one of several symptoms. It can also be caused by trauma to the eye (usually this results in the injured eye getting darker). Occasionally, people are just born with different colored eyes, and we presume that this is the result of variation in the expression of the eye color genes during embryonic development, although it isn't clear exactly what would cause such a variation in gene expression.

For a little bit more nitty gritty detail about heterochromia, you can visit these websites:

http://www.nlm.nih.gov/medlineplus/ency/article/003319.htm

http://en.wikipedia.org/wiki/Heterochromia

I hope this information is helpful. Good luck in genetics class!

Warm Regards,

Jen