Ryan Kramer's father is number 1058.

As one of more than 30,000 kids born each year through the assistance of a sperm donor, the 16-year-old counts that ID number among a handful of facts he will ever know about his biological dad. Born just outside Boulder, Colorado, where he lives with his mom, Wendy, Ryan has held on to the pieces of information 1058 put down on his form back when he was a student in Los Angeles in need of some cash: 6 feet tall, blue eyes, born in 1967, an engineering-degree candidate.

Ryan is even more curious about the man behind the number because Ryan is a math prodigy: He graduated from high school at 14, and is now a third-year aerospace engineering student at the University of Colorado. "I always loved math and hated the liberal arts," Ryan said. "And my entire maternal family is the polar opposite."

"How did he get this brain? That was one of our questions," added his mom. "There's that missing half of himself that he's questioned throughout his life."

Ryan understands that he has little hope of meeting number 1058. But when his sperm bank, California Cryobank, mentioned that there might be half-siblings -- between three and 30 of them, according to Ryan -- he decided to take action. "We decided that finding some of these half-siblings might be equally good at answering my questions," he explained, "seeing some of the invisible side of me in somebody else."

So the Kramers created the Donor Sibling Registry (DonorSiblingRegistry.com), a kind of social-networking site where donor families can connect in the hope of finding their half-siblings -- and maybe even their biological father. Using screen names to protect their privacy, users can browse by clinic or create a new posting with the little information they have about their respective sperm donors. To date, the site has nearly 7,000 members and has generated matches between 2,684 half-siblings. That's 2,684 kids discovering brothers and sisters they'd never even known about.

"It's shocking what it's turned into," said Ryan. "The first time we had a match between two siblings we were just awestruck! And now it's a daily event." Wendy said some people are overwhelmed at the discovery of these new relatives: "I get e-mails all the time like, 'Oh my God, I just came to the site and saw that I have three sisters. What do I do now? What do these people expect of me?' For some people, it starts out slow -- some e-mail, exchanging photos, gaining trust -- and for others, they're jumping on airplanes to meet each other."

Among the kids who have found each other through the site is a group of Denver teens: Tyler and McKenzie Gibson, 18 and 13; twins Rebecca and Erin Baldwin, 18; and Justin Senk, 16. The crew, who've come together over the last year, is a nature-versus-nurture case study. While they were born into different households -- Tyler and McKenzie to a single mom working in risk management, the twins into a Christian household with an ex-Air Force mom and Justin to a married couple with fertility problems -- they have donor number 66 in common.

While relationships with siblings you've been raised with can be loaded with an intense mix of affection and competition, the majority of the donor kids meeting up through the site have been thrilled to get a chance to know their brand-new siblings. The Denver crew was immediately struck by how familiar they seemed to one another. When Rebecca met her younger half-sister, she recalled, "I felt like, 'Omigosh, McKenzie, I know so much about you! I feel like I am you -- or was you." In Tyler, Justin saw himself reflected in an older brother he'd always wished he'd had. "With Tyler on the phone," he said, "we both thought there was an echo, because we sounded so much alike." The search is still ongoing for the Denver group: They've just discovered a new half-sister, 11-year-old Bree in Cleveland, who they'll meet this winter.

Artificial insemination has been in practice since the 1950s, but it was another 30 years until it became more commonplace. At this point, of course, it's a process that's best known in pop culture through fairly insensitive references to "turkey basters." But as an entire generation of donor kids is coming of age in the States, many of their families are wondering why they shouldn't know more about the men behind the numbers. With the majority of sperm donors still choosing to remain anonymous, some countries -- including, most recently, England -- have actually passed laws requiring that men reveal their identity if they are going to donate. Should we have a similar law in America, where the only federal regulation right now is a required donor HIV test?

The fact that the donor-sperm industry is unregulated allows the sperm banks to sell each donor's sperm to as many women as they choose. That means there are men out there who, whether they know it or not, have fathered literally dozens of children. The record on the DSR is currently 46 children by one man -- and that's only counting the kids who've registered on the site. Even if that donor wanted to meet the kids he's fathered, what kind of meaningful relationship could he have with 40-something little strangers? "Those 46 kids are never going to get to meet their donor simply because of their number," said Wendy, "and I think that's really sad."

While some donors would prefer not to have a relationship with the kids they've fathered -- after all, they chose to do so anonymously -- the DSR is making a difference. By creating the Web site, Ryan Kramer and his mom have given donor families a voice at a time when donor conception may be more relevant than ever. After all, the American family is getting a makeover: Single-parent households are on the rise, and gay couples are becoming parents in much greater numbers. Wendy even speaks about donor families' right to openness in the terms of the gay-rights movement. "It's like we're bringing something out of the closet that's been hush-hush for so long," she said with a laugh. "We don't want there to be any shame to how these kids were conceived."

"I think there is no road map here," she added. "This is uncharted territory."