Why does she have blue eyes and blonde hair if only one of her parents is not Japanese? Aren't those recessive traits?
Blonde hair and blue eyes are a recessive gene and usually in men the blonde hair goes away the older they get, sometimes even the blue eyes do. I had blonde hair and blue eyes as a kid, I no longer have either now.
Those traits aren't one way or the other. In actual hybrids you will not have black hair 100% of the time, in fact most of the ones I've seen have some sort of light brown hair.
If we can explain the genetics of this girl, I wonder how we'll explain the genetic background of characters with pink, blue, purple, green and orange hair, with matching coloured eyes to boot...
I don't even care about that, but Nisekoi honestly isn't that bad compared to some of the garbage that's been produced. There's no way you could've watched 50+ without stumbling over something worse.
It's easy, they were born with the main character gene. It gives you the traits of a potential main character. Filthy gaijin genes surpressed the eye and hair color attributes and inserted their own for her.