In the novel To Kill a Mockingbird, what are Aunt Alexandra's ideas about breeding and family? Why does Atticus tell them to forget it?

In Chapter 13, Scout mentions that Aunt Alexandra is obsessed with heredity and believes that each family has a "streak" in their ancestry. Alexandra maintains that several families throughout Maycomb have drinking streaks, gambling streaks, mean streaks, and funny streaks. Aunt Alexandra also believes that the longer a family has occupied a piece of land, the "finer" that family was. Aunt Alexandra criticizes Atticus for not teaching his children about the proud Finch family history. Atticus then attempts to have his children sit down and listen to him explain their family history. Scout and Jem can tell that Atticus is acting strange, and Scout begins to cry. Atticus feels silly telling his children to remember useless information and tells Scout and Jem to forget about it. He doesn't share the same enthusiasm towards family history that his sister does.