How does "Tom" treat Roxy? Why does Twain have "Tom" treat Roxy in this manner?

Tom views his mother his slave and chattel. This situation is dripping with irony. Roxy's own son treats her with disdain thinking himself above the woman that gave him life. The reader wonders why there is no natural connection for Tom. Twain is pointing out the whole nature versus nurture dichotomy in its purest form.