Gene testing accuracy has its limitations
DNA doesn’t reveal great detail about one’s origins, experts say
Maybe you got one of those find-your-ancestry kits over the holidays. You’ve sent off your awkwardly collected saliva sample, and you’re awaiting your results. If your experience is anything like that of me and my mom, you may find surprises — not the dramatic “switched at birth” kind, but results that are really different from what you expected.
My mom, Carmen Grayson, taught history for 45 years, high school and college, retiring from Hampton University in the late 1990s. But retired history professors never really retire, so she has been researching her family’s migrations, through both paper records and now a DNA test. Her father was French Canadian, and her mother (my namesake, Gisella D’Appollonia) was born of Italian parents. They moved to Canada about a decade before my grandmother was born in 1909.
Last fall, we sent away to get our DNA tested by Helix, the company that works with National Geographic. Mom’s results: 31 percent from Italy and Southern Europe. That made sense because of her Italian mother. But my Helix results didn’t even have an “Italy and Southern European” category. How could I have 50 percent of Mom’s DNA and not have any Italian? We do look alike, and she says there is little chance I was switched at
birth with someone else.
We decided to get a second opinion and sent away to another company, 23andMe. We opened our results together and were just as surprised. This time, I at least had a category for southern Europe. But Mom came back as 25 percent southern European, I only 6 percent. And the Italian? Mom had 11.3 percent to my 1.6. So maybe the first test wasn’t wrong. But how could I have an Italian grandmother and almost no Italian genes?
To answer this question, Mom and I drove to Baltimore to visit Dr. Aravinda Chakravarti of the Johns Hopkins University School of Medicine and the Bloomberg School of Public Health, who has spent his career studying genetics and human health.
“That’s surprising,” he told us when we showed him the results. “But it may still be in the limits of error that these methods have.”
The science for analyzing one’s genome is good, Chakravarti said. But the ways the companies analyze the genes leave lots of room for interpretation. So, he said, these tests “would be most accurate at the level of continental origins, and as you go to higher and higher resolution, they would become less and less accurate.”
As in my case — the results got me to Europe, just not Italy.
My 23andMe test also showed less than 1 percent of South Asian, Sub-Saharan African, and East Asian & Native American. This, Chakravarti said, is likely true because the genetics of people on a continental level are so different, and it’s not likely that South Asian will look like European. “Resolving a difference between, say, an African genome and an East Asian genome would be easy,” he said. “But resolving that same difference between one part of East Asia and another part of East Asia is much more difficult.”
I also learned that even though I got half of my genes from Mom, they may not mirror hers.
We do inherit our genes — 50 percent from each parent. But Elissa Levin, a genetic counselor and the director of policy and clinical affairs at Helix, says a process called recombination means that each egg and each sperm carries a different mix of a parent’s genes.
“When we talk about the 50 percent that gets inherited from Mom, there’s a chance that you have a recombination that just gave you more of the northwest European part than the Italian part of your mom’s ancestry DNA,” she said. That is also why siblings can have different ancestry results.
The companies compare customers’ DNA samples to samples they have from people around the world who have lived in a certain area for generations. The samples come from some databases to which all scientists have access, and the companies may also collect their own.
“We’re able to look at what are the specific markers, what are the specific segments of DNA that we’re looking at that help us to identify, ‘Those people are from this part of northern Europe or southern Europe or Southeast Asia,’ ” Levin said.