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 per cent of our DNA, they said. Yet five years later the human genome is being used to identify our racial differences and geographical origins - sometimes with dubious precision.

Several 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, the chat-show queen, 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.

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, and this was an area in which people should be aware of the reality of what they are being offered," the commission says.

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 recent 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.