First Jackie learned her brother Alex was her uncle. Then things got a little weird.
In the spring of 2012, the 34-year-old and her older sibling (their names have been changed) spit a few milliliters of saliva into plastic tubes and shipped them off to 23andMe, a personal genomics company, for consumer-grade scans of their DNA. Their family has a long history of cancer, alcoholism, and bipolar disorder, and Jackie, who happens to work for a biomedical research lab, wanted to learn all she could about her health risks and propensities.
Alex wasn't quite so into it. Jackie is curious by nature, the kind of person who's always asking questions. ("You never, ever, ever know anything for sure," she tells me. "As a kid, I was always saying, 'How do I know the sky is really blue?' ") Her brother's just the opposite: a military man with little time for facts that don't bear directly on his mission goals. Growing up, she called him "Robot," because he's so even-keeled and self-sufficient. When they both got emails saying that their genome data could be viewed online, Alex didn't open his.
The two agreed to look over their reports together, though. Meeting at their mother's place, they logged into 23andMe.com and checked the hints about their health and heritage that had been extracted from their genomes. None of what they found was so surprising or distressing, at least until they reached a screen that promised access to "close relatives" who might be in the system. Opting in, the siblings learned a fact about themselves that would have been disturbing, were it not so obviously in error: Jackie had an uncle, the software told them—an Uncle Alex.
When Jackie wrote about the glitch on the 23andMe message board, she got a quick reply: That's not an error, another user wrote; it means that you and Alex are more distantly related than you think. The scientists in Jackie's lab agreed: Full siblings have about one-half their DNA in common, as do parents and their children. But the genome scan must have showed that she and Alex share one-fourth of their DNA instead. That proportion could imply that two samples came from a niece and her uncle, or from a girl and her grandfather. But it could also mean that Jackie and Alex were half-siblings—that they shared one parent but not the other.
Alex, who is eight years older than his sister, refused to believe the news. When Jackie called him to explain what the "uncle" thing was all about, he snapped at her. She'd never seen the "Robot" so angry or distraught. "Mom did not cheat on Dad," he said. "It's a data-entry mistake. You're crazy!" But for Jackie, something had begun to click. She and her brother had never looked that much alike, and their personalities were opposite. Their parents had been separated for 20 years, and Jackie was never close with the man she’d always called her dad. Though he lived just 10 minutes down the road, they rarely talked at all. When she had a baby last year, he didn't even come to visit.
Jackie had sent in her DNA to learn something new about herself but ended up more confused than ever. That night, she went to her mother's house and heard about a one-night stand with a much older man, her biological father, now dead for many years. When she got home that night she went to the bathroom to wash off her makeup. "I didn't recognize myself," she says. "I looked in the mirror and thought, who is this person?"
Last December, 23andMe announced that it would be cutting prices for its genome scans. The 7-year-old company reduced the cost from $299 to $99, in the hopes of building a database of 1 million users by the end of 2013. (They’re one-quarter of the way there.) If that happens, how many of those clients will find themselves in the same dismaying situation as Jackie and her brother?
The study of false fatherhood, or nonpaternity, has turned up a wide variety of answers. University of Oklahoma anthropologist Kermyt Anderson says that measured rates of nonpaternity vary quite dramatically depending on the group of people being tested. Among those men who are quite confident of their status as biological fathers—the ones who volunteer their families for genetic studies of inheritance, for example—Anderson found a rate of nonpaternity of roughly 1.7 percent. At minimum, he says, 1 in 60 dads raises children that don't belong to him.
Anderson also went through data from companies that make their money testing for paternity. The men who send off DNA for these commercial tests presumably have cause to be suspicious. These men should have the highest incidence of nonpaternity, Anderson says. When he checked the research on this population, he found a median rate of close to 30 percent.