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New York - 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.

Things have changed. A decade ago, I was passing through Ben Gurion airport in Tel Aviv carrying a box filled with small tubes. Alerted by the Syrian stamp in my passport, the security staff gave me a hard time. After emptying my case, she asked what was in the box. I replied, irritably: “Arab spit”. “What?” she said. “I’m a geneticist, I explained, I have been sampling Palestinian DNA. At once, her face brightened – ah, DNA. Had I heard the then novel stuff on the shared male chromosomes of priestly Jewish families such as the Cohens? I had, and we parted on amicable terms.

The conversation gave me pause for thought. Joseph Mengele himself wrote his doctoral thesis on the relationship between jaw shape and racial identity. His ideas were pernicious rubbish and even 20 years later the thought of a genetic test for Jewish descent would have been treated with horror. Now, one has emerged and is not despised but hailed by many Jews themselves.

A scan of half a million variable sites across the genomes of several hundred Europeans and Americans, each aware from their family history of having had a recent Jewish or a non-Jewish ancestry, gave an absolute separation between Jews and others: even a single Jewish grandparent was enough to provide an unambiguous identity, written in DNA. A carefully chosen sample of just 300 of those sites does almost as well, and a test based on that would be cheap.

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.

For others, though, the story is murkier. A separate great centre of Jewish tradition and culture grew up in Spain. Most of the Sephardim arrived after the peninsula fell under Roman control in the second century BC. In 711 AD, a Muslim army invaded. The Jews flourished under a tolerant regime, often as lawyers, merchants and the like. Then the Church returned. After a century of persecution, they were expelled in 1492. The Sephardim were scattered over much of Europe, the Middle East, and the New World.

Their mitochondria, unlike those of the Ashkenazim, give no sign of a recent bottleneck. Their DNA show instead how porous the boundaries of faith may be. Threatened by the Inquisition, thousands of Spanish Jews left to places such as Turkey. Others converted, or pretended to do so – and one Portuguese village maintained a secret Jewish culture, marrying among themselves for five centuries.

Y chromosomes reveal much leakage across the religious divide. A fifth of all the male lineages of modern Spain are of Jewish origin, which means that millions of devout Spanish Catholics have Sephardic ancestry, while the Sephardim themselves, with their unique and ancient Jewish ritual, present a wider range of genetic variation than do their Ashkenazi cousins. Plenty of those with one faith have biological roots in the other. My wife, as it happens, comes from a Sephardic family and has relatives with surnames such as Cardozo and Pexiota. After 40 years here, she has still not got round to obtaining a British passport. In spite of the double helix, identity remains a confusing thing.

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