LATE last year, a 15-year-old boy rubbed a swab along the inside of his cheek, popped it into a vial and sent it off to an online genealogy DNA-testing service. But unlike most people who contact the service, he was not interested in sketching the far reaches of his family tree. His mother had conceived using donor sperm and he wanted to track down his genetic father.

That the boy succeeded using only the DNA test, genealogical records and some internet searches has huge implications for the hundreds of thousands of people who were conceived using donor sperm. With the explosion of information about genetic inheritance, any man who has donated sperm could potentially be found by his biological offspring. Absent and unknown fathers will also become easier to trace.