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Imagine never having met your father. He wasn't even present at your conception and doesn't know you exist.

Now imagine there are others who share this experience and might even share your DNA. One of these people could live down the block from you or pass you on the street. Would you consider them family?

The first significant wave of sperm donor children — an estimated one million people — has grown up with heavy questions on their minds.

Like adoptees, sperm donor kids are curious about their genetic origins. But unlike adoptees, these offspring could have upwards of 10 half-siblings who share their physical and intellectual characteristics.

"As much as people don’t want to think so, there’s a bond between these people," said Wendy Kramer, a single mother of 13-year-old Ryan, who was conceived by donor insemination.

In 1999, Wendy and Ryan started the Donor Sibling Registry to help link up biological siblings conceived from the same sperm donors. The site was inspired by the frustration they experienced when trying to find Ryan’s donor dad, known to them only as Subject 1058.

"I feel that there is a part of me, half of me, physically and emotionally that I can pull out from my mother’s side of the family," said Ryan. "Then there’s this other part that is invisible."

Ryan has known he was conceived through donor insemination since he was 2 years old and asked his mom why he didn’t have a father (Wendy and her ex-husband opted for donor insemination after having fertility issues, but divorced shortly after).

When Ryan was 7, he wrote to the California Cryobank, the sperm bank which helped his mother conceive, but they wouldn't provide anything more than what’s called a "long form" outlining the donor’s physical, medical and intellectual characteristics.

At the age of 18, Ryan can request that Cryobank contact his donor father, but until then, the clinic has nothing else to offer.

When Ryan turned 8, he learned that three families had asked for re-donations from Subject 1058. Since families can only request a particular donor if they've had a live birth from him, Ryan realized he had at least three half-siblings — but no means of finding them.

That’s when Ryan and his mother started the registry, which has almost 1,000 members, and has made more than 70 matches.

Wendy has been contacted by a woman who says she conceived two children from Ryan’s donor, but they haven't met because the kids don’t know they were conceived through donor insemination.

Secrecy is a common thread in the stories of sperm recipients and donors alike.

At age 24, Steve Martinez, now 46, became a donor at the Salt Lake Women’s Clinic after his human sexuality teacher offered male students extra credit for participating in the program, then writing a report on the experience.

"It was egocentric. I felt like I was a big, strong, good-looking guy and that if we were at the bar, she’d pick me anyway," said Martinez.

Between 1982 and 1984, Martinez donated roughly 30 times with no knowledge of what would become of his donations. He has signed on to several online registries, but hasn't found any of his offspring.

"It’s an incredible curiosity to me," he said. "Did they get my gray eyes, premature gray hair, do they like science, are they sports kids?"

The question of whether or not to connect donors and their offspring has yet to be aggressively addressed by the sperm bank community at large, but the issue is increasingly relevant.

"The industry is self-regulated," said Alice Ruby, executive director of the Sperm Bank of California. "The American Society of Reproductive Medicine recommends no more than 25 children from the same donor for a population of 800,000."

In a city the size of New York that could mean 250 offspring from one donor.

The self-imposed limit at the Sperm Bank of California is 10 families per donor worldwide, with each family free to produce as many offspring as they like from that donor, according to Ruby. But that sperm bank — the only non-profit one in the U.S. — is more conservative than most.

In 1983, the bank launched the first "donor identity release" program, which offers donors the option to sign up to be contacted when their offspring reach age 18.

"Everyone [sperm banks] followed our lead in providing some information on donors … very few followed us on donor identity release," Ruby said. "The conventional wisdom is to say most donors don’t want to be involved, and that men will not donate under donor release. But we’ve found that’s not true. Seventy percent of our donors opt for donor release."

To date, just 35 of the sperm bank’s donor identity release kids have turned 18, only five have come forward and only one has contacted her donor father. That number is set to explode in years to come.

"I think donor insemination is a very good thing for society. It can help out a lot, but there are still some things that need to be tweaked," said Ryan, a uniquely articulate 13-year-old. "I believe that it’s time to rethink the industry’s policies and their rules."