Submitted by Waspie Dwarf: DNA tests carried out on two British men have shed light on a mystery surrounding the ancestry of Thomas Jefferson, America's third president. 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. 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. Y chromosomes can be classified into broad groups (haplogroups) which, to some extent, reflect a person's geographical ancestry.

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