I recently finished Jacqueline Carey's Kushiel's Dart (which is not nearly the cheese-fest I expected, by the way), in which a woman is born into a certain career path by virtue of her eye color. Not in a let's-persecute-the-brown-eyes way, but in a "that color is linked to certain personality traits" way.
This is a common theme in fantasy, as many others have pointed out. Mieville even wrote a book deliberately to subvert it. Sometimes, one is chosen by genes or midichlorians or some inborn trait: Will in The Dark is Rising is born as a member of the Old Ones, so he never has to read What Color is Your Parachute. Other times, Gandalf comes along and says, "Yo! Hobbit! I've got a six-pack of dwarves and an adventure, and you're coming with me."
It's not just the heroes, though. In fantasy, nearly every character does what they are born to do, be it farming their family's land or ruling the kingdom. In fact, not following the family roles has to be explained away. In McKinley's The Blue Sword, Damarians follow roles their class and parents expect - unless they see visions when drinking special water. In Spindle's End, the princess isn't the princess, because of magical interference. (This is a book that manages to simultaneously uphold and subvert the notion of destiny.) In many books, the only reason a peasant rises high is because he is actually of royal birth. And the rest of the peasants aren't unhappy they're peasants, because that just suits them.
Partially because of this, fantasy (especially epic fantasy) has been called reactionary and conservative. No doubt there are some fantasy fans who long for the day when men were men, women were beautiful, and you could tell a wizard by the color of his aura. But I think it also speaks to people who don't long for the good old days, to people who appreciate the work of suffragettes, the anti-slavery movement, the Civil Rights movement, feminism, and queer theory. In our culture, we are dazzled by choice, and we long for certainty.
Make no mistake, the numbers clearly show that Americans have less class mobility than previous generations, but we First Worlders still have more choices than most people throughout most of time. Not only can we choose careers, but we can dress goth or preppy or hippie, we can get married young or never, we can take up cricket or softball. (Here, I am indebted to recent conversations with my sister about why young people aren't going to church; only 15% of young adults who attended some kind of services as kids will return to any church as adults. Having choices is only one of many reasons, but it's not an insignificant one.)
I mean, look at me. I studied different things as an undergrad, masters student, and doctoral student. I've worked in jobs with no clear connection and considered others seriously. (Did you know, there was a period when I even thought about the Coast Guard? That would have been a disaster.) The idea of knowing, for certain, that "this is what I'm meant to do" sounds very comforting. The thought of a system that naturally gives everyone satisfying life choices a cozy (and impossible) dream.
Back to the book that sparked this: I liked the book, and I have no desire to hold it up as a particularly egregious example. As Carey has pointed out, the book is actually intended to subvert certain tropes. And you can't subvert all the tropes at once, or the original pattern is lost altogether. (And that's a whole other post.)