Putting the Gene Back in Genealogoy

“That’s just ludicrous!” yelled Martin Richards when I read him the e-mails from Family Tree introducing me to my Genetic Cousins. “It’s not meaningful to say you’re related to those people any more than you’re related to anyone in America, just because you have a common ancestor in the Ice Age. If you trolled the world that way you’d find more relatives than you’d know what to do with.”

This doesn’t mean the company is lying when it says there’s a 99.9 percent chance I’m related to those Genetic Cousins, although it may be using a broader definition of “related” than its many prospective customers might assume. With each generation, the number of ancestors a person has increases by a factor of 2 to the power of the generation. Go back five generations, adding each generation together, and you’ve already got 62 direct ancestors—not counting siblings, cousins, aunts or uncles. At Family Tree DNA, the best-case scenario gives you a 50 percent chance that you and your Genetic Cousins share a common ancestor within seven generations—a pool of 254 relatives—and a 90 percent chance you share a relative in 23 generations, or 16,777,214 ancestors. That is to say, if you gathered enough people to populate the state of Florida today, then added about 778,000 more, you’d likely (but not surely) find a distant relative in common with your Genetic Cousin. “If you just took any two random people and looked at their DNA,” says University of London geneticist David Goldstein, “you would find some part of their genomes that was derived from a common ancestor fairly recently.” And by “fairly recently,” he’s talking less than 23 generations back. “The question is, Does it actually say much about the similarity of people if they share DNA 15 to 20 thousand years old? No, it absolutely, emphatically doesn’t.”



This is not the first time Goldstein has encountered a gap between the way these tests are marketed and their underlying science. In 2000 he found that within one small island population, people of Viking descent had a higher frequency of a certain genetic sequence than the non-Viking population. One company now tests for that sequence, saying people with it descended from Vikings. “We simply never said that,” says Goldstein. “People sometimes write me saying, I have the Viking gene, what does that mean? I say, I haven’t the slightest idea. Outside that tiny island, we have no clue what that sequence means.”

It’s tempting to shrug off misrepresentations and dramatic claims as the sort of harmless hype that often floods to market when a truly significant new technology or scientific discipline debuts. Goldstein, though, isn’t so quick to shrug. “I really worry about a backlash,” he says. “Because there is something quite serious we need to do with genetics and health care in the very near term.” Goldstein envisions a time in the near future when doctors will prescribe medications based on patients’ genetic profiles. “In order for us to use genetics in personalizing medicines,” he says, “it’s quite important that the public be able to trust the way genetic information is used. If we’ve oversold dramatically what we can tell people about their heritage using genetics, then there’s naturally going to be skepticism about what we can tell them about their responses to drugs.”

When pressed, Greenspan concedes that mitochondrial DNA isn’t much use for traditional genealogy. “It’s projecting back to a group that may have formed 20,000 to 30,000 years ago,” he told me. “So that’s more of an anthropology test.” As for its use in genealogy research: If two women came to him saying, Are we related? he could give them an answer with traditional DNA mapping techniques. “Other than that,” he said, “if you’re looking to find your family through a female DNA test, you may as well be playing the lottery.”