By Paul Rincon
Science reporter, BBC News
DNA tests carried out on two British men have shed light on a mystery surrounding the ancestry of Thomas Jefferson, America's third president.
DNA results from Thomas Jefferson were a mystery
In the 1990s, DNA was taken from male relatives of Jefferson to see if he fathered a son with one of his slaves.
They found the president had a rare genetic signature found mainly in the Middle East and Africa, calling into question his claim of Welsh ancestry.
But this DNA type has now been found in two Britons with the Jefferson surname.
Professor Mark Jobling, from the University of Leicester, and colleagues discovered the two British Jeffersons possessed the same rare male (or Y) chromosome type as the third US president.
Genetic analysis showed the British men shared a common ancestor with Thomas Jefferson about 11 generations ago. But neither knew of any family links to the US.
The unusual lineage has not been found in white Britons before. This discovery scotches any suggestion that Jefferson - who was president between 1801 and 1809 - must have had recent paternal ancestors from the Middle East.
Last month, Professor Jobling's group reported the discovery of seven white men from Yorkshire carrying a West African Y chromosome.
The Y chromosome is a package of genetic material passed down from father to son, more or less unchanged - just like a surname.
Over many generations, it does accumulate small changes in its DNA sequence, allowing relationships between different male lineages to be studied.
THE DNA MOLECULE
The double-stranded DNA molecule is held together by four chemical components called bases
Adenine (A) bonds with thymine (T); cytosine(C) bonds with guanine (G)
Groupings of these "letters" form the "code of life"; there are about 2.9 billion base-pairs in the human genome wound into 24 distinct bundles, or chromosomes
Written in the DNA are about 20-25,000 genes which human cells use as starting templates to make proteins; these sophisticated molecules build and maintain our bodies
Y chromosomes can be classified into broad groups (haplogroups) which, to some extent, reflect a person's geographical ancestry.
Certain haplogroups might be common in, for example, East Asia but rare in Europe. In Britain, sharing a surname raises the likelihood of sharing the same Y chromosome type.
The two men in the latest study had paternal ancestry in Yorkshire and the West Midlands respectively.
Thomas Jefferson's haplogroup - shared with the two men from Britain - is known as K2.
K2 makes up about 7% of the Y chromosome types found in Somalia, Oman, Egypt and Iraq. It has now been found at low frequencies in France, Spain, Portugal and Britain.
Of the K2s looked at by the study, Jefferson's Y chromosome was most similar to that of a man from Egypt. But genetic relationships between different K2s are poorly understood, and this may have little significance.
Instead, say the researchers, their study makes Jefferson's claim to be of Welsh extraction much more plausible.
Professor Jobling told BBC News: "Finding that Jefferson's Y chromosome was one mutational step away from an Egyptian type makes you think 'crikey, could he have a relatively recent origin in the Middle East?'
"Our point is that we find, at lower frequencies, French, British and Iberian K2s and they are jolly diverse. His fits into that picture of a west European sub-population of K2."
Hemings was a slave at Jefferson's Monticello estate in Virginia
The DNA sequences of individual K2s - including those from Europe - are quite different from one another.
This "genetic diversity" has to accumulate over time, supporting the idea that Jefferson's haplogroup is not a recent introduction into Europe.
The haplogroup has probably been present for centuries in the "indigenous" population of western Europe, says Professor Jobling, and is not exclusive to the Middle East and Africa.
It could have been introduced to Europe by the first modern humans to colonise the continent 40,000 years ago.
Another theory concerns the Phoenicians, an ancient maritime trading culture that spread out across the Mediterranean from their home in what is now Lebanon. K2 is relatively common in Lebanon, leading to suggestions that European K2s may be descendents of these ancient traders.
In 1998, Jobling and others completed an investigation looking at whether Jefferson, main author of the Declaration of Independence, fathered a son with Sally Hemings, a slave he owned.
Rumours had long existed that they had one or more children. Since Jefferson had no legitimate surname-bearing progeny, the team used samples from descendents of his paternal uncle.
They compared these with descendents of Eston Hemings Jefferson, Sally's last son. The Y chromosomes matched, suggesting Jefferson, or one of his paternal relatives, was Eston's father.
Details appear in the American Journal of Physical Anthropology.