The Human Stain (2003) is an excellent example of the implications of the one-drop rule that only people of African decedent have historically faced in America. It details the great lengths the main character went through, out of fear of facing discrimination, to avoid having his African heritage revealed. He spends the majority of the movie actively trying to “pass” (a key sociological term in the race literature) as White. See a more detailed summary of the movie here http://www.imdb.com/title/tt0308383/plotsummary
The one-drop rule, that the main character of the movie struggles to avoid, is a term that historically and presently applies to people who have any African heritage. This “rule” states that if a person has one drop of African blood they are defined as Black (FYI African American is no longer considered PC, because of a group of immigrants from the West Indies. Check out Mary Water’s Water’s Black Identities for more details on this fascinating group).
Historically this rule was backed up by laws throughout the United States and even though it is not a rule that is enacted by law, it is still supported by social customs. For example, president Obama, who is half White and half Black, selected Black as his race on the U.S. Census (see http://articles.latimes.com/2010/apr/04/nation/la-na-obama-census4-2010apr04). This suggests that the rule is still present today and that even people of African descent use the rule to define themselves as Black.
For more details on this fascinating subject, I highly recommend James Davis’ book Who is Black?: One Nation’s Definition published in 2001.