Successful blacks

Black Harvard prof Henry Louis Gates Jr writes in the NY Times:

I have been studying the family trees of 20 successful African-Americans, people in fields ranging from entertainment and sports (Oprah Winfrey, the track star Jackie Joyner-Kersee) to space travel and medicine (the astronaut Mae Jemison and Ben Carson, a pediatric neurosurgeon). And I’ve seen an astonishing pattern: 15 of the 20 descend from at least one line of former slaves who managed to obtain property by 1920 — a time when only 25 percent of all African-American families owned property.

Someone observed:

There's nothing "astonishing" [his word] about Gates's statistic. If we go back to great-grandparents (as Gates does in his Winfrey example), then four relevant families exist (by hypothesis) in 1920. At that time, according to Gates, 25% of black families owned property. The probability that "at least one" of the four relevant antecedent families held property in 1920 is thus 1.0 - (0.75 x 0.75 x 0.75 x 0.75) which is about 68%. Gates's sample of 20 yielded 75%.

He is correct. Gates's argument is bogus.