But personality has nothing to do with genetics...

I don't think it's his personality that they're comparing. It's their mental disorders. Mental health can definitely come from parents or your background. My anxiety came from my dad, for example.

The little boy is neurotic and a hypochondriac so that doesn't have to do with his personality. But as far as their personalities go, they're not too similar, IMO

Actually, it's never been established how much of a person's personality is nature and how much is nurture, but it's certainly some combination of both depending on the individual. Brain chemistry absolutely plays a role in behavior and personality, and there is much evidence for brain chemistry and "hard wiring" being inherited traits such as predisposition for certain behaviors and tendencies such as depression, for example. Yes, how a child is raised also influences behavior and personality, but as parents of multiple children can tell you (even multiple birth children such as twins or triplets), they each have their own personality and react to and are affected by things in completely different ways. Much of the end result is in the hands of parents, but much of is is not.

I'm a firm believer that we're products of our environment and as we get older we have a choice about what type of person we want to be. This, however, is just a movie, so I couldn't care less.