http://en.wikipedia.org/wiki/One-drop_rule
The one-drop rule is a historical colloquial term in the United States that holds that a person with any trace of African ancestry is considered black unless having an alternative non-white ancestry which he or she can claim, such as Native American, Asian, Arab, or Australian aboriginal.[1] It developed most strongly out of the binary culture of long years of institutionalized slavery.
During the Black Pride era of the Civil Rights Movement, the stigma associated with sub-Saharan ancestry was turned to a socio-political advantage.
The one-drop rule and its consequences have been the subject of several works of popular culture. In the musical Show Boat, Steve, a white man who is married to a black woman, is pursued by the sheriff, who is going to arrest Steve and charge him with miscegenation. Steve pricks his wife's finger and swallows some of her blood. When the sheriff arrives, Steve asks him whether he would consider a man to be white if he had "negro blood" in him. The sheriff replies that "one drop of negro blood makes you a negro in these parts". Steve tells the sheriff that he has "more than a drop of negro blood in me". After being assured by others that Steve is telling the truth, the sheriff leaves without arresting Steve.
In the Family Guy episode "Peter Griffin: Husband, Father...Brother?," Peter discovers that he has a pre-Civil War black ancestor. Because of the one-drop rule, Peter, who has always thought of himself as white, self-identifies as black and starts to attend events that draw primarily African-American audiences.
The Public Enemy song "Fear of a Black Planet," from the album of the same name, neatly summarizes this phenomenon with the lines: "Black man, black woman, black baby / White man, white woman, white baby / White man, black woman, black baby / Black man, white woman, black baby."
The Pencil test
During the system of apartheid in South Africa, one drop of sub-Saharan blood was not enough to be considered black. South African law maintained a major distinction between those who were black and those who were coloured. When it was unclear from a person's physical appearance which racial classification they belonged to, the pencil test was employed. This involved inserting a pencil in a person's hair to determine if the hair was kinky enough for the pencil to get stuck. If the pencil remained stuck in a person's hair, the person was "black."
_________________
Alis volat propriis
State Motto of Oregon