Who’s Your Daddy?

The unintended consequences of genetic screening for disease

A few months ago, I sat down at my desk to open a letter that could tell me whether my father was really my father. In fact, that letter could tell me whether the men going back 10 generations on my paternal side were the biological fathers of their children.

I wasn’t caught up in some bizarre multigenerational paternity suit. A scientific officer at a genetic testing company knew that I was interested in genealogy, and he had offered to run my DNA through a sequencer. A few weeks earlier, I’d swished mouthwash inside my cheeks, sealed the mouthwash in a tube, and mailed the tube to the company.

My doughty Scandinavian ancestors passed the test. My DNA revealed no obvious instances where the man named on a birth certificate differed from the man who was my biological ancestor. But I was lucky. Many efforts to trace male ancestry using DNA terminate at what geneticists delicately call a “non-paternity event.” According to Bennett Greenspan, whose company, Family Tree DNA, sponsors proj­ects that attempt to link different families to common ancestors, “Any project that has more than 20 or 30 people in it is likely to have an oops in it.”

The law of unintended consequences is about to catch up with the genetic-testing industry. Geneticists and physicians would like us all to have our DNA sequenced. That way we’ll know about our genetic flaws, and this knowledge could let us take steps to prevent future health problems. But genetic tests can also identify the individuals from whom we got our DNA. Widespread genetic testing could reveal many uncomfortable details about what went on in our parents’ and grandparents’ bedrooms.

The problem would not loom so large if non-paternity were rare. But it isn’t. When geneticists do large-scale studies of populations, they sometimes can’t help but learn about the paternity of the research subjects. They rarely publish their findings, but the numbers are common knowledge within the genetics community. In graduate school, genetics students typically are taught that 5 to 15 percent of the men on birth certificates are not the biological fathers of their children. In other words, as many as one of every seven men who proudly carry their newborn children out of a hospital could be a cuckold.

Non-paternity rates appear to be substantially lower in some populations. The Sorenson Molecular Genealogy Foundation, which is based in Salt Lake City, now has a genetic and genealogical database covering almost 100,000 volunteers, with an overrepresentation of people interested in genealogy. The non-paternity rate for a representative sample of its father-son pairs is less than 2 percent. But other reputed non-paternity rates are higher than the canonical numbers. One unpublished study of blood groups in a town in southeastern England indicated that 30 percent of the town’s husbands could not have been the biological fathers of their children.

Even with a low non-paternity rate, the odds increase with each successive generation. Given an average non-paternity rate of 5 percent, the chance of such an event occurring over 10 generations exceeds 40 percent.

Most people can’t look that far back on their family trees, but I can. Someone on the Olson side of my family once spent an inordinate amount of time tracing the family’s male lineage. My relative’s genealogical research indicated that my father’s father’s father’s father’s father’s father’s father’s father’s father’s father migrated from Finland to Norway in the middle of the 17th century. If that is the case, I have a particular connection to that man.

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