|MadSci Network: Genetics|
The quick and easy answer is: yes, it is indeed possible for people to have eyes of two different colors. It's observed fairly frequently in some inbred strains of dogs, like malamutes, but is rare in humans. A few famous examples are : David Bowie, Kiefer Sutherland and Christopher Walken. I didn't find any diseases associated with this condition, so aside from social effects I guess it's perfectly harmless. With the advent of colored contact lenses, now it's possible for anyone to have two different colored eyes.
I wish I could explain to you how or why this occurs naturally, but I can't find any relevant research, nor even mention of the condition in ophthalmologic texts. The basic biology of eye color is quite simple: eye color is the result of production of the brown pigment melanin in the cells of the iris. The amount of melanin determines eye color; lots of melanin produces brown eyes, some melanin produces hazel eyes, and little melanin produces blue eyes. (Green eyes weren't mentioned, but I suspect they are a combination of blue with another pigment.) Genes controlling the production of melanin therefore influence eye color.
An interesting possibility that could account for eyes of different color in the same individual is somatic mosaicism. This is probably easier to describe than explain: in multicellular organisms, every cell in the adult is ultimately derived from the single cell fertilized egg. Therefore every cell in the adult normally carries the same genetic information. However, what would happen if a mutation occurred in only one cell at the two cell stage of development? Then the adult would be composed of two types of cells: cells with the mutation and cells without. If a mutation affecting melanin production occurred in one of the cells in the cell lineage of one eye but not the other, then the eyes would have different genetic potential for melanin synthesis. This could produce eyes of two different colors.
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