Definitely Maybe (film)/Headscratchers

Headscratchers from the film Definitely Maybe:

 * While it's certainly not outside of the bounds of possibility, does anyone else think it mildly odd that the brown-haired man and the blonde-haired girl wind up with the red-haired daughter? I was almost entirely certain that the hair color was going to be a clue as to who was the mother right up until the end.
 * It could be possible that Will had red hair, but it got darker when he got older. Or, the mother dyes hers. Or maybe they both have Irish heritage.
 * More probably Scottish, as contrary to popular belief, Irish genes seem to present more commonly in pale, freckly blondes or brunettes. Just a pet peeve.
 * Well, red hair is a recessive trait to the more dominant trait of being a blonde or a brunette. Makes the reveal more interesting?