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| Gene's dubious precision in tracing
roots |
Steve Connor
June 30 2005 at 07:37AM
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London - When scientists announced the completion of the Human Genome
Project at a press conference in June 2000, they went out of their way to
extol the unity of humankind. We all share more than 99.9% of our DNA,
they said. Yet five years later the human genome is being used to identify
our racial differences and geographical origins.
Companies have
sprung up offering testing services that claim to be able to trace our
genetic roots. In Britain they offer to identify which of seven ancient
European clans we are descended from. In America they promise to find out
whether a person has Jewish, African, European or native-American genes.
Oprah Winfrey has even had a test. "I went in search of my roots," she
says, "and had my DNA tested, and I am a Zulu."
Sometimes the aim
of those wanting a test is to fill a void in their personal history, which
can be especially painful for the descendants of the African slave trade.
Others have wanted to complete a genealogical search of their family, a
hunt that may have come to a dead-end using conventional paper trails.
Some Americans have even paid for DNA tests in the hope of
financial gain. If they can prove they are descended from certain Native
American tribes they can claim a share of profits from casinos on tribal
lands.
| Human DNA is stored in 46 chromosomes
arranged in 23
pairs | Britain's watchdog on genetics, the Human Genetics
Commission, says some claims made in the name of "genetic genealogy" can
be misleading.
"The scientific information that genealogy tests
could provide might not be as precise as some of the companies suggest.
Tracing family roots has traditionally relied on using genealogical
records including certificates of births, marriages and deaths, which
allow the tracing of relationships using surnames that are passed down
through the male line. But the advances in DNA analysis have offered a new
possibility of tracing our ancestors through the sequence of chemical
"letters" that make up the alphabet of our genetic code.
If two
living people share a similar DNA sequence, it could mean that they once
shared a common ancestor.
Human DNA is stored in 46 chromosomes
arranged in 23 pairs, yet just one of these chromosomes is particularly
useful in tracing origins. All chromosomes except the male Y-chromosome
engage in swapping or recombination of DNA fragments between each
chromosome in the pair. This mixing produces a patchwork of ancestral DNA
on a single chromosome, making it hard to decipher your ancestors based on
analysing this DNA alone.
A much better tool, for men at least, is
the Y-chromosome that determines maleness. Men inherit just one such
chromosome from their fathers and, being on its own in the cell, it does
not get involved with the messy business of recombination. A boy's
Y-chromosome is more or less identical to that of his father, his paternal
grandfather, great-grandfather and so on.
| Few records of Zulus being involved in the
Atlantic slave
trade | Analysing the genetic sequence of the Y-chromosome is
therefore an excellent tool for looking at male ancestry. There is now a
thriving business doing just this, especially for Americans trying to
trace their family roots in Europe.
The problem, says Professor
Mark Jobling of Leicester University is that although such tests may
indicate a degree of relatedness, they do not provide accurate information
on how far back the common ancestor lived. "Dating is limited. Saying that
two men share recent patrilinear ancestors is possible, but showing when
they shared that ancestor is more problematic."
Another useful tool
for tracing human origins is mitochondrial DNA. This is the only DNA that
occurs outside the cell nucleus, and is passed on from mothers to both
their daughters and sons. Again there is no recombination to mix up the
mitochondrial DNA sequence, allowing scientists to trace ancestors back
many generations. But unlike the Y-chromosome, both men and women have
mitochondrial DNA, which permits ancestral tracing for both sexes.
Just as the Y-chromosome has shed light on the early evolution of
men, mitochondrial DNA has revealed the origins of maternal ancestors
going back many thousands of years. In fact scientists have even located a
notional "mitochondrial Eve", the last common female ancestor of all
humans, who lived some 200 000 years ago.
The same sort of
research has shown that there are about 10 broad divisions of
mitochondrial DNA in Europeans today. Each category is descended from a
single common female ancestor who lived between about 15 000 and 30 000
years ago.
Oxford Ancestors, a company set up by Professor Bryan
Sykes of Oxford University, offers tests which place people in one of
seven mitochondrial DNA "clans" to represent what Professor Sykes has
romantically dubbed "The Seven Daughters of Eve".
"It's all part of
increasing your sense of belonging in the world at a time when people feel
increasingly disconnected," Sykes says. "It's not something you'd think
was intrinsically useful." Nevertheless, nearly 20 000 people have each
paid about £180 for a DNA test carried out by Oxford Ancestors.
However, not every geneticist is happy with the idea of using
mitochondrial DNA to suggest definite ancestral origins for individuals.
"This information can tell us about the process of colonisation to paint a
more detailed portrait of Europe's past but mitochondrial DNA is only a
tiny fraction of our genome," says Martin Richards, a geneticist at Leeds
University.
"Overall, members of a so-called 'clan' are no more
likely to be more closely related to each other than to other members of
their population."
Every DNA test used to trace our ancestral
origins has limitations. The first is simply based on the fact that if we
go back far enough, we are all related to one another.
The other
limitation of a DNA test is that it focuses on a relatively short element
of your entire genome.
In addition, when companies such as DNAPrint
Genomics offer to match your DNA with a racial or ethnic group, the
accuracy with which they can do this relies on the quality of their
genetic database, which can be poor.
Say Professor Jobling: "And
it's difficult for companies offering such tests to say that this is going
to cost you £150 but it's not going to be worth very much."
This
may well turn out to be true for Oprah Winfrey. She believes she's a Zulu
from southern Africa because her DNA test says so. Yet there are few
records of Zulus being involved in the Atlantic slave trade, which mainly
focused on tribes from west Africa. Whatever the DNA sequence that linked
Oprah with Zulus, it may yet turn out to be present in west Africans.
Although DNA tests have proved invaluable in identifying close
relatives, highlighted by paternity tests, they have proved problematic in
finding our distant cousins. - The Independent
- This article was originally published on page
10 of Cape Times on June 30, 2005
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