What “streak” does Atticus tell Aunt Alexandra that the Finch family must have in To Kill a Mockingbird?

Atticus tells Alexandra that the Finches must have an incestuous streak.

Apparently every family in Maycomb has some kind of “Streak.”  Alexandra suggests that Stephanie Crawford inherited her nosiness.  Atticus decides to let her know how he feels about the Streaks.

Atticus said, “Sister, when you stop to think about it, our generation’s practically the first in the Finch family not to marry its cousins. Would you say the Finches have an Incestuous Streak?”  Aunty...

Atticus tells Alexandra that the Finches must have an incestuous streak.

Apparently every family in Maycomb has some kind of “Streak.”  Alexandra suggests that Stephanie Crawford inherited her nosiness.  Atticus decides to let her know how he feels about the Streaks.

Atticus said, “Sister, when you stop to think about it, our generation’s practically the first in the Finch family not to marry its cousins. Would you say the Finches have an Incestuous Streak?”  Aunty said no, that’s where we got our small hands and feet. (Ch. 13)

Heredity is very important to Alexandra.  She believes that the Finches are superior because they have owned the same land for a long time.  Atticus tries to explain to Alexandra that heredity isn’t everything, but Alexandra wants Scout and Jem to understand the importance of their family name and where they fit in Maycomb’s social hierarchy.

Atticus is uncomfortable trying to explain the concept of good breeding to his children.

Our father was actually fidgeting. “No, I just want to explain to you that—your Aunt Alexandra asked me… son, you know you’re a Finch, don’t you?”

“That’s what I’ve been told.” Jem looked out of the corners of his eyes. …

Atticus crossed his knees and folded his arms. “I’m trying to tell you the facts of life.” (Ch. 13)

By “the facts of life” Atticus means that Scout and Jem are the product of generations of “gentle breeding.”  In other words, they are part of Maycomb’s upper class.  With the trial starting, Alexandra thinks it is important that Scout and Jem understand where they fit into Maycomb society, despite the insults sent Atticus’s way.  Since Alexandra came to help, Atticus decides to humor her.

Atticus does not care about heredity or breeding. He has taught his children that value comes from a person’s character, not his family name.  Yet he has no wife, so he feels like he has to succumb to Alexandra’s nagging.  It is not something that he actually believes.