Who is a Jew? DNA can hold the key

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Who is a Jew? As the recent passport row shows, that question can be murky, with elements of belief, values, descent and nationality mixed in.

It also has dark reminders of a terrible time in history when Jewish blood meant death; and science, or pseudo-science, claimed to be able to sniff it out.

Judaism is inherited down the female line – as are mitochondria. Their DNA shows that today's Jews from the largest group, the eight million Ashkenazim – most of whom once found their home in central and eastern Europe, and who now represent the majority of American Jews – have few grandmothers. Around half descend from just four women who bear mitochondrial types found almost exclusively in that population. Two million trace their descent from just one of those ancient predecessors.

In 1650, there were only 100,000 Ashkenazim in Europe, a number then further reduced by pogroms. In 18th-century central Europe, though, came massive expansion of that population, largely because of their relatively good living conditions. In Frankfurt, Jewish life expectancy was at aged 48, compared to 37 among non-Jews. By 1800, Jews numbered two million and by 1900 almost four times as many.

Much of the growth occurred in the Rhine Valley – modern-day Germany. The increase was concentrated among a few well-off families, many of whom had 10 children while the poorest classes had far fewer. As a result, the majority of today's Ashkenazim derive from a small proportion of that population, two million from one mother, quite literally their shared Eve, who probably lived – unknown and unrecognised – in an affluent household in a German or Polish village three centuries ago. A shared close identity through mothers, grandmothers, and more is, for millions of Ashkenazim, a genetical fact....

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