April 18, 2012

All one family

From the Herald

It is one of the most improbable family connections. One is an actor famed as a languid Lothario. The other was one of the world’s most brutal but brilliant military leaders. Nevertheless geneticists say their analysis shows Tom Conti is indeed directly related to Napoleon Bonaparte.

For this to be true you need to stretch the usual meaning of “directly related” a little. More than a little.  In fact, using that definition, pretty much everyone in the world is probably related to Napoleon, so you also need to stretch the usual meaning of “improbable”.

What the story actually says is that Tom Conti and Napoleon have the same Y-chromosome haplogroup.  That is, they have a common great-great-…-great-grandfather in the very distant past, and so share a small chunk of DNA.   The Y chromosome is special only because it’s easier to track ancestors; Tom Conti will also share small chunks of DNA on other chromosomes with many other people, based on a common ancestor that wasn’t solely in the male line.

In our genetic research, we measure millions of common genetic variants on large numbers of people. Every one of those variants started off as a mutation in a single person, who is an ancestor of all the people who now carry the variant. For the variants we are interested in, this is at least 1% of all people with European ancestry.   These people are all related, in the Conti-Napoleon sense. And that’s just looking at one genetic variant out of millions.

When you go back as few as 30 generations, you have a billion ancestors, which is more than the number of people alive at that time. There has to be a lot of overlap and double-counting and it’s not at all surprising if there is overlap between your billion ancestors and Napoleon’s.  Or Winston Peters’s



Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »