Ah, but it is magic, and it does not have to follow the rules of science and genetics. Even if it does, some things are caused by environment, not genetics, particularly the in utero environment, can have a huge impact. We do not know if say there was some True Love interacting with the impervious spell that Rumpel put on Snow that caused Emma to be what she became.
From a writing and story standpoint, making Henry the overpowered kid hero trope could be very cheesy.
“If I had a world of my own, everything would be nonsense. Nothing would be what it is, because everything would be what it isn't. And contrary wise, what is, it wouldn't be. And what it wouldn't be, it would. You see?” -- Lewis Carroll, Alice's Adventures in Wonderland & Through the Looking-Glass