Dad?

Human geneticists testing people for heritable diseases quite frequently stumble across cases where the father of record cannot be the biological parent. Genetic counselors have a rule of thumb that these discrepancies, known delicately as nonpaternity cases, will range from 5 to 10% in an average American or British population. For the US population as a whole, "The generic number used by us is 10 percent," said Bradley Ppovich, vice president of the American College of Medical Genetics.

Source: Wade, Nicholas. Before the Dawn: Recovering the Lost History of Our Ancestors. Penguin, 2006.