The One Drop Rule, was coined in early 19th Century America as a term for racial classification. Used to institutionally segregate Blacks, the One Drop Rule dictated that any presence of African American blood in ones lineage made you in both the eyes of Society and the Law Black.
However in our time this One Drop does not separate us. We through science understand that all Humanity can be traced to Africa. No, not everyone is African or Black explicitly. But throughout every corner of the Planet we are all connected.