"Fredo had nothing from the get-go" because poor Fredo was never given the chance Boy, oh, Boy didn't Fredo show us all?!
What did Vito do, to guide, mentor, nurture and embrace his different sons?
Vito's fatherly views! of his sons – Thanks! Pop
I think that it's a premise of both the book and at least the first movie that the sons' qualities are inborn. Michael is not just rational - he's representative of the rational man. He's both guided and bound by his intellect. Likewise, Sonny does his emotional things because he's emotional by nature.
I'll submit that Fredo represents the physical man in the novel. Feed him, sex him, and he's happy. Pointedly, he actually does pretty well for himself in the novel, as he's alive at the end and has a "legitimate" job. Coppola changes that Freddy into the first movie's Fredo, and endows him with his father's "sweet nature." Having assigned Fredo that role, Coppola seems to apply the rule of "nice guys finish last" to him, and just knocks him around for three hours.
