Reposted from:
http://www.nytimes.com/2007/10/21/weekinreview/21basic.html?ref=science

Two curious political relationships of Senator Barack Obama came to light last month in The Chicago Sun-Times: he is an 11th cousin to President Bush, and a 9th cousin once removed to Vice President Dick Cheney. The Bush-Obama ancestors are Samuel and Sarah Soole Hinckley of 17th-century Massachusetts, The Sun-Times wrote; the Cheney-Obama ancestors are Mareen and Susannah Duvall, 17th-century immigrants from France.

The story sent new ripples through the political world last week when Lynne Cheney repeated Mr. Obama's link with her husband in a television interview promoting her new book. It's not as if Mr. Bush and Mahmoud Ahmadinejad were found to be identical twins separated at birth; still, the genealogy was incongruous.

But the Bush-Obama-Cheney nexus means rather less than it may seem. Conventional genealogies conceal how rapidly genes get diluted down the generations. You inherit directly only half of your father's genes, a quarter of each grandfather's, an eighth of any great-grandfather's. And with ancestors much further back, you may share no more genes than you do with a random stranger met in Times Square.

That may be why many people cannot trace their family tree very far back: perhaps intuitively, they realize they don't share much, genetically speaking, with those distant forebears.

Tracing ancestry through the father's surname, a surrogate for the Y chromosome, may seem absurd given that the Y carries just 70 of the 25,000 or so genes in the human genome. But the compilers of Burke's Peerage could be engaged in a quest less ludicrous than it might sound. In in-bred populations like Europe's nobility, just as in isolated villages, everyone dips from the same little gene pool. In such cases, you inherit only half of your father's genes directly, but of the half that got left on the cutting-room floor, some may have come to you via your mother, who shared them with your father via a joint ancestor.

But in out-bred populations, like that of the United States, where people generally marry people who are not related to them in any known way, genes get diluted really fast. Mr. Obama probably inherited a minute fraction — one divided by two to the 11th power — of Mareen Duvall's genome, which would amount to less than one gene, assuming the Y chromosome was not inherited. Much the same would be true of Mr. Cheney. The chance that they inherited the same one gene is vanishingly small. So the fact of their genealogical relationship, whatever its political symbolism, is genetically meaningless.