My Y chromosome turned out to be as Finnish as sautéed reindeer—I al‑ most certainly inherited it from that 17th-century Finnish émigré. But even if my Y chromosome had turned out to be suspiciously un-Finnish, I probably could have come up with a story to protect my legitimacy. I could have said that my Finnish ancestor was the descendant of a Mongolian invader, or the son of a trader from Istanbul, or even a Spanish diplomat fallen on hard times (though in fact I know that he was a peasant farmer). I could have said that one of the men in my paternal lineage was adopted after his mother and father died. The imagination is a wonderful balm for bruised expectations.

But genetic tests don’t lie, which means that our imaginations may be in for a workout. For example, groups of people in many parts of the world trace their lineage to particularly prominent male ancestors. In some cases, genetic tests reveal a kernel of truth behind these stories. Genghis Khan’s Y chromosome really is widely distributed in Asia, for instance. Still, many of these stories have social rather than genealogical roots. “Many times we romanticize about the different groups that we have ancestry with,” says Rick Kittles, a geneticist at the University of Chicago who founded the company African Ancestry. When Kittles has told clients that their genetic tests don’t coincide with what they believe, a few, he says, have been shattered.

Frankly, I hadn’t thought much about these issues before sitting down to open that letter from the genetic testing company. If I had, I doubt I would have agreed to the test. If my Y chromosome was not what I expected, would I tell other family members about it—including my teenaged son? Would I have been tempted to encourage my brother, then my male cousins through my father’s brothers, then my male second cousins through my grandfather’s brothers, and so on to be tested so that I could determine where the non-paternity occurred? I think we’d all have been better off assuming the best and shunning the test.

But the pressure to undergo genetic testing is about to increase. New technologies are reducing the cost of sequencing DNA. Researchers are now establishing extensive databases of DNA sequences combined with health information so they can link specific genes to diseases. And once the contributions of our genes to common diseases are discovered, everyone could benefit from DNA testing. Already, the Personal Genome Project at Harvard University is seeking volunteers who are willing to have their DNA sequences and medical information posted on the Web for biomedical purposes, even though the project warns that a person’s DNA could be used to “infer paternity or other features of the volunteer’s genealogy.”

Two of the men most responsible for the sequencing of the human genome—James Watson and Craig Venter—are making most of their genomes available on the Web. But if their sons ever decide to have their DNA tested, they could face the same situation I did in opening that letter. Watson has kept part of his genome private because he doesn’t want his sons and the public to know whether he has a genetic variant predisposing him to Alzheimer’s disease; he seems unconcerned about what the rest might reveal.