Researchers studying Thomas Jefferson's Y chromosome have found it belongs to a lineage that is rare in Europe but common in the Middle East, raising the possibility that the third president of the United States had a Jewish ancestor many generations ago.

No biological samples of Jefferson remain, but his Y chromosome, the genetic element that determines maleness, is assumed to be the same as that carried by living descendants of Field Jefferson, his paternal uncle. These relatives donated cells for an inquiry into whether Jefferson had fathered a hidden family with his slave Sally Hemings, a possibility that most historians had scoffed at.

But researchers reported in 1998 that the Jefferson family chromosome matched perfectly that of a male line descendant of Eston Hemings, one of Sally Hemings' sons. The genetic evidence was not conclusive by itself but made a strong case combined with the historical evidence that Hemings had indeed become Jefferson's mistress after the death of his wife, Martha.

Geneticists at the University of Leicester in England, led by Turi King and Mark Jobling, have now undertaken a survey of the branch or lineage to which Jefferson's Y chromosome belongs.

Jefferson's Y chromosome belongs to the branch designated K2, which is quite rare. It occurs in a few men in Spain and Portugal and is most common in the Middle East and eastern Africa, being carried by about 10 percent of men in Oman and Somalia, the geneticists report in the current issue of the American Journal of Physical Anthropology.

Puzzled at the lack of K2 Y chromosomes in Britain, given that Jefferson's family traced its origin to Wales, Jobling's group decided to scan a special population most likely to carry K2 -- that of men named Jefferson. Of 85 British Jeffersons tested, just two proved to have Y chromosomes of the K2 lineage. The paternal grandfather of one was born in Yorkshire, that of the other in the west Midlands.

Discovery of these two English members of K2 supports the idea that Thomas Jefferson's recent paternal ancestry is from Britain. Had they not been found, Jobling's team writes, the geographic distribution of K2 would have made the Middle East seem the most likely origin of Jefferson's family.

The fact that K2 is common in the Middle East, however, raises the possibility that Jefferson had a Jewish ancestor, Jobling said. Jewish Y chromosomes resemble those of Middle Eastern peoples, and the Jewish diaspora is one way Middle Eastern chromosomes entered Europe. But because so little work has been done on the rare K2 lineage, "our research raises the possibility, but doesn't help anyone to answer it either way," Jobling said.

Michael Hammer, a geneticist at the University of Arizona, said he had compared the Jefferson Y chromosome with those in his database of Y chromosomes and found close matches with four other individuals. There was a perfect match to the Y chromosome of a Moroccan Jew, and matches that differed by two mutations from another Moroccan Jew, a Kurdish Jew and an Egyptian.

Hammer said he would "hazard a guess at Sephardic Jewish ancestry" for Jefferson, although any such interpretation was highly tentative. Sephardic Jews are descendants of those expelled from Spain and Portugal after 1492.

Bennett Greenspan, president of Family Tree DNA, a DNA-testing service, said that among the 90,000 Y chromosome samples contributed to his database, K2 occurred in 2 percent of Ashkenazim, Jews of central or northern European origin, and 3 percent of Sephardim. "Whether the non-Jews with K2 are descendants of Jews or come from an earlier migration into Europe is hard to say, but my sense is that it's separate migrations from the Middle East," Greenspan said.