July 01, 2009

The genetics of history & Thomas Jefferson

Kevin Gutzman alludes to genetic evidence pointing to a strong likelihood that Thomas Jefferson fathered at least one of Sally Hemings’ children. Why strong likelihood as opposed to 100% certainty? The genetic test in question focused on the Y chromosome, which is passed exclusively through males, and Jefferson naturally shared his Y chromosome with his brother. So it is certain that some of the descendants of Sally Hemings derive from the Jefferson lineage, but not necessarily Thomas Jefferson. But there’s a way to establish full certainty: obtain a sample of Thomas Jefferson and Randolph Jefferson’s genetic material from the family burial ground. Genetic material has been obtained from 40,000 year old Neandertal finds, so I presume that men buried ~200 years ago would be a relatively easier. The goal would be to compare the total Y sequence of the Jeffersons and the Hemings who are presumably descended from them. For “government work” brothers do share identical Y chromosomes passed down from their fathers, but, every human has a unique suite of new mutations which are distinctive. So there should be identifying genetic mutations on the Y chromosome which can be used to separate the putative descendants of Thomas and Randolph Jefferson.

On a broader note, this principle means that many people do in fact have good evidence of their descent from one particular man centuries in the past, Lord Somerled. We also now have circumstantial evidence now in relation to the putative descendants of Genghis Khan and Muhammad (through Ali and Fatima). One point to keep in mind in relation to the lines of Hemings who were eliminated from descent from the Jeffersons: a fraction of individuals in any given patrilineage are often of another paternity. So there might have been an interruption of the line. In rare surnames there is often a pattern whereby ~50% of the men are descendants from the same man (what one would expect), but ~50% are descendants of hundreds of other men. This is evidence of the “interruption” process over the generations.

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