Young Black Glamour Girl

A considerable portion of the U.S. population identified as black actually have some Native American or White ancestry. For instance, genetic studies of African American people show an ancestry that is on average 17–18% European.
• One drop rule
Historically, the United States used a colloquial term, the one-drop rule, to designate a black person as any person with any known African ancestry. Legally the definition varied from state to state. Thomas Jefferson had slaves who were legally white (less than 25% Black) and legally slaves (mother was a slave). Outside of the US, some other countries have adopted the practice, but the definition of who is black and the extent to which the one drop "rule" applies varies from country to country.
The one drop rule may have originated as a means of increasing the number of black slaves and been maintained as an attempt to keep the white race pure. One of the results of the one drop rule was uniting the African American community and preserving an African identity. Some of the most prominent civil rights activists were multiracial, and advocated equality for all.