Danielle Pagano and JoEllen Marsh have never met. Nor have they ever seen the father they share. They know him only as Donor 150.

The two teenage girls belong to a new type of family model where half-siblings born from the same anonymous sperm donor to different mothers have tracked each other down using the internet.

The Donor Sibling Registry is redrawing family relationships for the hundreds of children born through artificial insemination and desperate to know more about their backgrounds and themselves.

Ms Pagano, 16, from New York, will join Ms Marsh, 15, at her home on Thursday just as their unconventional family tree looks set to grow further.

After reading media reports at the weekend, a third teenage girl came forward yesterday. Her biological father was also Donor 150 from the same clinic, the California Cryobank.

Wendy Kramer, founder of the Donor Sibling Registry, told The Times that the largest new-style extended family established through her website consisted of 12 mothers of 16 children all made possible by “Fairfax Donor 647”.

Ms Kramer, whose own son, Ryan, was conceived thanks to donated sperm, said: “All of these people are redefining the family. What does it mean to be connected? What does it mean to be family?”

The website, founded five years ago, now has 6,049 members, including 24 children from Britain, with 1,067 halfsiblings matched.

Thousands of children have been born with donated sperm and many yearn for contact with siblings and their biological father.

New Scientist reported this month how a determined teenager circumvented the code of anonymity in Britain by matching his own DNA to an internet database of Y chromosomes, which led him to a family surname.

Despite all the new family ties that have been created, one of the most poignant stories highlighting some of the complex issues behind tracing biological relatives concerns Ms Kramer’s own 15-year-old son, whose curiosity encouraged her to start the website. She knows that Ryan has eight biological siblings and suspects that there could be as many as 15 given that less than half of women report their births back to the sperm clinic. But so far contact has not been made with any. Ms Kramer said: “A mother of two of his half-sisters contacted us, only to tell us the next day that she did not tell her daughters they were donor conceived and probably never would.

“He knows about his half- sisters but it is kind of ironic that the kid who started the website is the bittersweet story.

“We know there are a lot more out there for Ryan and we hope they come forward.”

Ms Pagano and Ms Marsh are now considering a visit to the registered birthplace of Donor 150 in Wilmington, Delaware.

The only other things they know about him come from his sperm bank profile: he was married with two children, 6ft, weighed 163lb with blond hair and blue eyes, and liked acting, animals and yoga.

Ms Pagano described the moment that she finally made contact with her new sister.

“The first time we were on the phone, it was awkward.

“I was like, ‘We’ll get over it’ and she said, ‘Yeah, we’re sisters’. It was cool.”

WHO'S THE DADDY?

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