^Key word, "supposedly." As I said, just because one character believes that's the case, that doesn't make it objectively true.
I mean, really, what does it mean to "act human?" Human behavior encompasses everything from Gandhi to Hitler, from Mother Theresa to Jack the Ripper. It hardly makes sense to assume that human genes would make an individual act less aggressive or more timid. As Captain Kirk was known to say, we're predators at heart, as much as the Klingons are, but we've chosen to control it.
So Laneth's statement is clearly absurd, simply a manifestation of her own racial prejudices and self-doubt. She believed
that being part-human made her weaker, and that made her insecure. It wasn't a genetic change that made her more susceptible to fear, it was her loss of self-confidence.