LIKE many children conceived with the help of an anonymous sperm donor, Ryan Kramer grew up knowing he might never learn the identity of the man who was his genetic father. But that did not stop him wondering if he had any half-brothers or sisters that the same man might have sired.

The 15-year-old’s quest for knowledge about possible siblings inspired his mother Wendy to create a fast-growing internet support group that is helping to redefine the American family.

Across the country, groups of children born of different mothers but linked by a shared sperm donor are finding each other online. Many are gathering for novel family meetings. In most cases, the only thing they know about the donor they have in common is the number he was given by a sperm bank — in Ryan’s case, donor 1058.

“Your whole notion of family goes out of the window when you see these children meet for the first time,” said Wendy Kramer, who recently hosted a gathering of five children born of three different mothers all impregnated by sperm from a Denver doctor’s donor 66. Among the children were McKenzie Gibson and Rebecca Baldwin, 17, who found that they looked just like each other.

In the Washington area, a group of 11 women who bought sperm from donor 401 at the Fairfax Cryobank in Virginia have made contact online and are planning to meet up once their children — still mostly babies — are older.

At the California Cryobank, two daughters fathered by donor 150 found each other through Kramer’s Donor Sibling Registry and met for the first time last year.

Danielle Pagano, 16, of Seaford, New York, was bitter when her parents first told her the truth about her conception three years ago. Yet her subsequent discovery that she shared her genetic father with JoEllen Marsh, 15, of Russell, Pennsylvania, has eased her anger and provided her with a lifelong friend.

“The first time we were on the phone, it was awkward,” Pagano told The New York Times. “And she said, ‘Yeah, we’re sisters’ . . . it was cool.”

According to Kramer, almost 1,500 of the 7,000 people registered on her website have found matches with shared donors. The biggest number of matches to a single donor so far is 22.

There are now estimated to be more than 1m children in America who were conceived with donated sperm, and more than 30,000 others are born each year. By contrast Britain has had 25,000 in the past 15 years.

As the children age and become more interested in the circumstances of their birth, donor offspring are beginning to present a formidable challenge to a sperm distribution industry that in America remains largely free of government controls.

Unlike Britain, where recent changes to the law allow donor children to trace their biological parents, most US sperm banks still guarantee anonymity and provide little help to groups such as Kramer’s.

“You guys in England got it together on this,” said Kramer. “You built a registry just for your country and your government funds it with thousands of pounds because in the UK you saw value in this.”

She added: “My site has grown beyond my wildest expectations, I’ve got 7,000 people on it, and hardly a day goes by without people matching up. But I’m still working as a volunteer and I can’t find anyone to give me $10 to run it.”

The debate on donor children’s rights took an unexpected turn last year when a 15- year-old boy took a swab from the inside of his cheek, sent it to a DNA-testing agency, then cross-referenced the results with online genealogical databases to track down his genetic father.

The boy’s brilliant detective work stirred renewed interest in the plight of donor children who are not always so lucky in their quests for a missing part of their identity.

It took Kramer and her son three years before word-of-mouth and news coverage attracted enough people to her website for success stories to emerge. And there was jubilation in the Kramer household when Ryan’s appearance on a television news programme two years ago was seen by a woman who e-mailed to say she had recognised him instantly.

She said Ryan looked just like the two daughters she had conceived with donated sperm, and the match was quickly confirmed. “It was the best day for my son,” Kramer recalled. “It was his 13th birthday and he had two half-sisters he had never known.”

But there was to be no happy ending for this group of sperm siblings. The woman sent an e-mail that the children could not meet. She and her husband had decided never to tell their two daughters they were donor-conceived. “For Ryan, it was ‘Oh my gosh, I’ve got two half-sisters but they might never know I exist’,” said Kramer. “He was crushed.”

Kramer is convinced honesty and disclosure are the only fair options for the children of sperm donors. “We’ve come so far in the world of adoption,” she said. “We know truth is best. We know as a society it’s better not to lie to children. But in the world of donor conception, we’re not there yet.”

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