A teenager born from an anonymous sperm donation has managed to track down his biological father using a swab of saliva and the internet, in a case that could throw donor anonymity into disarray.

Scientists have given warning that men who have given sperm are now at risk of being traced by offspring with the dramatic growth of genealogy and DNA databases on the internet.

They said that guarantees of identity secrecy, which have been given to thousands of donors over the years, could prove worthless with the increase in such resources.

The boy, who was 15 at the time, was able to evade anonymity safeguards by sending a swab of saliva taken from the inside of his cheek to a website used by genealogists to chart their family trees.

For a fee of $289 (£163), his DNA was mapped and added to the website’s database, which sends out email alerts to its customers when close matches appear.

The service, FamilyTreeDNA.com, compares the user’s Y chromosome — which passes from father to son virtually unchanged — against a database of Y chromosomes from other men.

Nine months later the teenager, an American who has not been named, was informed of two close matches. He was contacted by the two men, who were both using the website to trace their family trees.

Though the biological father had never supplied his DNA to the site, his Y chromosome profile, shared by his son and closely matched by the two other men, suggested they must be related.

In fact, the similarities in Y chromosomes between the teenager and the two other men revealed a 50 per cent chance that all three had the same father, grandfather or great-grandfather.

According to New Scientist, which is publishing the report this week, both men who contacted the teenager had the same surname, although with different spellings.

Using this information, he then used a second website, Omnitrace.com, to compare the surname with the few details of his biological father given by the fertility clinic, which include date and place of birth, and his college degree. The search brought up a match for his father.

Similar details of British donors, as well as religion and occupation, are offered to their genetic offspring.

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