Caroline Seawright wrote:

Seeing that the children don't get to spend much time with their
father, the mother is the head of the family to the children. And
so in Ranma 1/2, it's probably a way of satirising this. Although
Genma raised Ranma for ten years of his life, Ranma's still more
interested in his mother, and he only shows love towards her, and
only disrespect towards Genma. Maybe he's upset that Genma stole
him away from the mother-child relationship that he should have
gotten?

Ranma has several other causes for resentment.  Not least of which
is having to steal or fight to keep from losing every scrap of food
he eats.  And since it's the mother's job to teach children to be
respectful and polite in the first place, without a mother, Ranma is
naturally disrespectful and impolite.