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.

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.

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20, 2010

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