Mark D. Shriver, a molecular
anthropologist at Penn State University, has studied population with a team of researchers. In 2002 they published results of a study regarding the racial admixture of Americans who identified as white or black.
Shriver surveyed a 3,000-person sample from 25 locations in the United States and tested subjects for genetic make-up. ... In his study, of those persons who identified as black, their total ancestry reveals 18% white ancestry, the equivalent of having 22 white ancestors among the 128 5xgreat-grandparents. About 10% of blacks have more than 50% white ancestors.
Population studies by researchers other than Shriver have found that in general, blacks had an average white ancestry of 25–30%. I got the 20-30% from a sentence in the paragraph you just read.
Obama's ancestry:
http://kennethelamb.blogspot.com/2008/02/barak-obama-questions-about-ethnic.html
Mr. Obama is 50% Caucasian, that from his mother. What those who want Mr. Obama to write history by becoming "America's first African-American president" ignore is that his father was ethnically Arabic, with only 1 relative ethnically African Negro - a maternal great-grandparent (Sen. Obama's great-great grandparent, thus the 6.25% ethnic contribution to the senator's ethnic composition.).
That means that Mr. Obama is 50% Caucasian from his mother's side. He is 43.75% Arabic, and 6.25% African Negro from his father's side.