In Pudd'nhead Wilson, Roxy, a young slave woman, switches her fair-skinned baby with Master Driscoll's. Only she is able to tell them apart. Her action has huge implications later in the novel when Tom, who is actually her son, grows up to be lazy and conniving as the Master's son; his character is spoiled and selfish and ultimately kills his uncle in an argument.
One part of the novel that would differ is that Wilson would not have had his dramatic revelation in the courtroom about Thomas Driscoll's fingerprints. He makes the discovery that the babies had been switched at birth, and eventually the fake Tom is sold down the river. The outcome of the novel would have been completely different in that respect.
Of course, answering the posted question also depends on the old nature versus nurture debate. Does Roxy's baby grow up to be lazy and selfish because of his genetic makeup, or was it more of an influence of his spoiled upbringing? In this case, environment has shaped and influenced the upbringing of "Tom," causing him to be a shallow and basically selfish individual.