What are aunt Alexandra´s ideas about breeding and family? Why does Atticus tell them to foget it? Who do you think is right?

Aunt Alexandra believes that traits are passed down through the bloodline, something that defines a family as good or bad. The Finch family, of course, is one of the good familes..... and as such, the children should behave accordingly.

Atticus, who is able to look beyond the family name, tells the children to forget it. Atticus' perspective is the more accurate.... a family name does not determine the value of an individual.