Kingsblood Royal. Sinclair Lewis

Neil Kingsblood is a white middle-class man who discovers, while researching his family background, that he is directly descended from an African adventurer on the American frontier. Through various machinations, Kingsblood loses his banking job and takes a lesser one. He begins to be treated differently by former acquaintances, despite the lack of visible black African ancestry. He is forced to choose between continuing what he has come to see as a hollow existence in the white community and taking on the oppressed minority status of the black community. After Kingsblood tells several white friends about his newfound ancestry, the news quickly spreads, and he finds that acquaintances change their behavior toward him. He engages in a quixotic struggle against the racism newly apparent but widespread in his community.