At the Centre for Genetic Anthropology, University College London, biologists have been tracing the legacy of Y-chromosomes in the genetic material of modern and ancient peoples living beside the Red Sea. Because it passes virtually unaltered from father to son, the Y-chromosome can help answer historical questions.

"Tourists, soldiers, sailors and traders frequently leave their Y-chromosomes behind," says Neil Bradman, the centre's chairman. By studying patterns in Y-chromosome DNA, he hopes to map the migration of different peoples.

The centre is focusing on the exodus of the Jews from Palestine in the 1st and 2nd centuries AD. Comparison of Y-chromosomes taken from men in Jewish communities around the world has already yielded interesting results.

Although Jewish identity is usually defined matrilineally, membership of the Jewish priesthood is patrilineal. The UCL researchers have found that the Y-chromosomes of the Cohens (as the priests are often named) show a paternal line that is different to that of other Jews. Remarkably, the same lineage was found in the priests of both main Jewish communities, the Ashkenazis and the Sephardis - groups separated geographically for more than 1,000 years.

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Now the researchers plan to tackle the identity of the Jews of Eastern Europe - when and how they got there - about which almost nothing is known. "One theory supposes that the Jews of Poland came from the Rhineland speaking Old German, which acquired Slavic words by contact with the local Slavs and Hebrew because of the Jewish literature," says Vivian Moses, the centre's director.

"Another theory holds that the Jews came from Italy across the Alps and acquired their German in Switzerland and Austria. Yet another says that they came from the Balkans speaking some proto-Slavic language. There is even the idea that there were large-scale conversions of some Slavs to Judaism at that sort of time."

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Moses hopes to test these ideas by comparing the Y-chromosomes of East European Jews with those of other European men.

There are sometimes unexpected consequences to such research. After the publication of their study of the Cohens, Moses was inundated with mail. "A large number of people have written to us asking to be tested to show that they're not Jewish priests, because that places restrictions on them," Bradman says. "They cannot marry a convert ora divorcee. There are some people who would like tobe able to show that they are not Jewish priests."