In the latest X-Men movie, Professor Xavier called heterochromia a mutation. Turns out this condition, where your eyes are two different colors, is a little more complicated than that.
Heterochromia can mean having each eye be a different color. It can mean one of your eyes has a patch with a different color in it. It can be there from birth, or can grow over the years. And often it has nothing to do with mutation.
If an eye gets pigment at all, it's due to melanocytes - special cells that pump out the pigment melanin. A lot of melanocytes means a brown eye. A few means green or hazel. Very few means a blue eye. The melanocytes don't develop in the emerging eyes, though. They travel to the eyes. They travel because they're told to travel from a variety of complicated sources, any of which can fail. If the melanocytes from one eye or one part of an eye don't get told to travel due to damage to the brain or some kind of delay in the signal, they don't go anywhere, leaving one eye lightly pigmented. Alternately, they can travel to an eye but get destroyed or damaged one site. This again results in different pigmentation.
Then there is the periodic darkening of the eye. The eye is just a big ball of goo. Things can seep into it. When injuries occur, matter can settle in the eye and darken it. And then there's Bowie Eye. Singer and goblin king David Bowie got in a bar fight and sustained damage to one eye. One pupil is permanent dilated, so the eye looks darker than the other.
But what about actual mutation? Well, it can happen. One gene mutation causing heterochromia is seen in Waardenburg syndrome. The modified genes disrupt melanocyte movement in all kinds of areas of the body. People with Waardenburg syndrome have patches of skin that are different colors as well as different eye color. (It also is associated with different degrees of deafness and a slightly different shape to the head.) Different colored eyes can also be caused by chimerism, when two fertilized eggs fuse to become one embryo. The different sets of genes continue acting on their own instructions, but work together to make one person, who can have a blue right eye and a brown left one. Lastly there is one of the rarest causes of heterochromia, one set of genes in one cell mutating early fetal development and guiding the development of that part of the body.
So while Xavier was talking a good game in that movie, it's not just mutation that changes eye color. (I like his speech better, though. Very groovy.)
Image: Xavier Nájera