The Name Behind the Number One child's efforts to find the man whose sperm donation gave him life.

July 10, 2007  -- Ryan Kramer, 16, is one of the smartest teenagers you will ever meet. He's already skipped ahead four grades and is now a junior at the University of Colorado in Boulder.

He completes math problems that fill-up entire blackboards, just like Matt Damon's character in the film "Good Will Hunting." While math and science problems are a breeze for Ryan, it's a really basic question that has stumped him for years.

Ryan's mother, Wendy Kramer, became pregnant with him 17 years ago using sperm from an anonymous donor. Ryan says he began asking about his father from a very early age.

"I was 2 years old at the time and going through preschool and seeing other kids who had two-parent families. I approached my mom and said, 'So did my dad die or what?'"

In the best way she could, Wendy explained that she did not know his father and that they may never know him because he had been promised anonymity.

Still, when Ryan's curiosity remained strong a few years later, Wendy suggested that her son write to the sperm bank requesting more information.

Looking for Answers

According to Wendy, "Donors sign up for anonymity. That's their choice, but I also honor my child who never signed an agreement with anybody. The question is, why does a donor's rights trump the child's rights every time?"

All Ryan knew was that his father had been assigned a number, but the sperm bank let it slip that his donor also had fathered other children. Ryan now believed he had half-siblings … and he came up with a way to find them.

Ryan and his mother started a Web site called the Donor Sibling Registry. If you know your donor's number and the bank from which the sperm was procured, you provide the Web site with that information and you'll find out whether you have any matches. Thousands of people have found half-siblings and in some cases, the actual sperm donor.

Even though Ryan knew he could have dozens of half-siblings, it would be years before he would make a match, and even then it would not end up the way he wanted.

'Everything Conceivable'

Liza Mundy, a Washington Post writer and author of the book "Everything Conceivable: How Assisted Reproduction Is Changing Men, Women and the World," said she believed there could be more than 1 million offspring of sperm donors in the world. "There's nobody tracking it. There's nobody counting," said Mundy.

Surprisingly, Mundy says, women using donated sperm is not a new idea. "For more than 100 years, sperm donation has been conducted surreptitiously. If a couple was suffering from male infertility, they would go to a doctor and he would say, 'Well, here is a solution and don't say anything to anyone about it.'"

So is there a limit to the number of children one donor can "father?" Scroll through the Donor Sibling Registry and you'll see some donors who have been responsible for 20 or more births. Because all of the information on the Web site is put there voluntarily, the actual number for many donors could be much higher.

"There's no regulation," Mundy said. "Some of the banks say we'll cap it at 10 offspring, but if the pregnancy is not reported — and many times it's not — then the sperm bank has no way to know whether or not a child has resulted."

Todd Whitehurst knows of at least seven children born from his sperm donation back when he was a graduate student at Stanford. He answered an ad in the student paper because he thought it would be a good way to make a few extra dollars, but he also says he did it because he thought it was a nice thing to do. "I really felt like I could make a difference in people's lives."

Whitehurst had a background that many potential parents found appealing. "I have bachelor's degrees and master's degrees in electrical engineering and computer science … then I went to medical school and got an M.D. from U.C.-San Francisco."

While not the only "father" listed on the Donor Sibling Registry, Whitehurst is in the minority when it comes to donors who want to meet their offspring.

"I don't think many donors even want to be contacted. I don't really understand that. To me it seems like an amazing opportunity to meet these children that resulted from donations," he said.

But what about 16-year-old Ryan Kramer? Three years passed between the time he launched the site and his first match. The mother of two girls contacted Ryan through the site. She let Ryan know that the same donor who had fathered him was responsible for her two daughters.

However, Ryan's joy at the news soon ended. Yes, he learned that these two young girls were his half-sisters, but as far as their mother was concerned he would never get to meet them. She told Ryan she and her husband had kept the truth of their conception by a sperm donor a secret from the girls and planned on keeping it from them.

Ryan was crushed that a family secret was keeping him from the only other family members he knew he had other than his mother. Just how long would it be before Ryan made the connection he so desperately wanted to make?

ABC News will answer that question and will be with Todd Whitehurst as he meets three of his offspring for the very first time on "Primetime."