Since most fictional worlds are independent of one another, one tends to assume that, unless otherwise stated, each world has roughly the same incidence of such conditions as the real world. Still, it's kind of funny to think, say, how many more characters with albinism I've read about than I've met. Or probably even seen. There's an even bigger gap for heterochromia - I've only ever met one person that I can think of who had it (and it wasn't obvious, e.g. one blue eye and one brown, but more of a one green/one hazel thing).
In some stories, these things are the result of magic or other supernatural forces. This can change their significance in the story - a person's natural heterochromia might be used as a symbol for some kind of internal conflict, whereas maaagical heterochromia might have more practical implications ("The blue eye sees your thoughts!"), or be otherwise telling ("She's been like that ever since the spell backfired. We don't know what else might have changed").
The way I see it, there are four big reasons why certain rare conditions appear commonly in characters in fiction. And - here it comes - KA-LIST!
- Because it's symbolic. This came up in the YALSA listserv's discussion of heterochromia, which could probably win an award among rare physical characteristics for Most Potential for Symbolism.
- Because it's the point. It may not be the point of the whole character, or of the whole story, but the author genuinely wants to explore some aspect of the lives of people with this condition, or the condition is otherwise responsible for the character's being in the story.
- Because it happens. Some argue that, in the same way that you should never have to justify a character being a certain gender, orientation, race, etc., you shouldn't have to have a reason that the character has a more unusual descriptor. Others argue that this is an excuse to use rare characteristics as gimmicks. I think that, like many things in writing, it's all in how it's handled. (Vague enough for you?)
- Because it's cool. This may be a secondary motive for an author who would more readily cite one of the other reasons.
(This is assuming that the state in question is still rare in the world of the book. If it's, say, a futuristic novel in which people readily alter their eye colors or the whole world has for some reason become albinistic, all bets are off.)
I know there are conditions besides albinism and heterochromia that pop up way more frequently in fiction than real life, but I'm blanking on them. Anyone?