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These siblings took DNA tests and got different results. Why determining ancestry is rarely accurate.

Determining your precise ethnicity under any circumstances is iffy. seems to try as hard as anyone to get it right.

Bob Carden, left, and his sister Barbara, right.
Bob Carden, left, and his sister Barbara, right.Read moreFamily photos

My wife bought the DNA test for my birthday. Great gift, cheap, easy and fun. Everyone is doing it. Six weeks after sending in the spit, I got the results. Nicely presented — colorful graphs and charts — but they seemed a bit off and more vague than I had expected.

For instance, my grandfather was an immigrant from Greece. He came through Ellis Island in 1911. I figured that made me at least 25 percent Greek. Yet, Ancestry showed a range of 9 to 30 percent Italy/Greece, with a likelihood of 18 percent. Vague in itself — not to the mention the fact that mixing Italians with Greeks would have driven my grandfather to the nearest bottle of Metaxa.

Nevertheless, I thought little more of it until I mentioned the test to my sister Barbara. Unbeknownst to me, she'd had the test done, as well, and was awaiting the results. They came a week later. And those results were more than just a bit off.

Our Italy/Greek numbers were about the same. But her results showed 37 percent British Isles, whereas I was 53 percent. She also showed 10 percent Scandinavian and 7 percent East Europe — neither of which even appeared in my results. And we're about as Scandinavian-looking as Jennifer Lopez.

Let's chase the elephant out of the room. The odds that my happily married and saintly mother might have had a dalliance, with, say, a Norwegian sailor back in the 1950s are nil.

So, same parents, different DNA test results. What gives?

"It's very difficult to accurately find your ancestry under any circumstances," said Jonathan Marks, an anthropology professor at the University of North Carolina at Charlotte. "There has been genetic mixing for thousands of years. These tests are fun but rarely accurate — 10 percent Scandinavian could be no Scandinavian because the test could very easily be 10 or 15 percent off."

The imprecisions are not limited to Ancestry. My sister's son took a DNA test from 23andMe, another popular DNA testing company. His father is Italian and Irish, named Rettaliata. The test showed no Greek and just 1 percent Italian.

"The methodology they use in determining the DNA markers is solid," said Deborah Bolnick, an anthropology professor at the University of Texas. "The challenges come with interpreting those DNA sequences to say something accurate about your ancestry."

Those challenges sure aren't conveyed in the ubiquitous TV ads. Folksy testimonials from happy customers beam with wonder. "I found out I was 16 percent Italian," says one consumer. Well, maybe you are and maybe not. Experts say assigning a specific percentage is problematic.

"You are dealing with probabilities here, not certainties," said Sarah Tishkoff, professor of genetics at the University of Pennsylvania.

Our genes identify many characteristics about us — for instance, there is a gene that programs eye color. Genes contain DNA, the chemical basis of heredity. A genetic test is never going to be 100 percent accurate in determining ancestries tied to specific ethnic groups. Ethnicity is not a trait derived from a single gene or a combination of genes. If only it were that simple.

Determining your precise ethnicity under any circumstances is murky. Ancestry seems to try as hard as anyone to get it right.

About my sister and me having different results, Ancestry explained that it is not all that uncommon for siblings with the same parents to have different ethnic traits.

"Each parent gives 50 percent of their DNA to you, but the subset of DNA you get could be different from your sibling," says Eurie Hong, senior director of genomics, research and development at Ancestry.

Bolnick agrees that siblings could potentially inherit some different DNA sequences from a parent. But she is a bit skeptical that this could lead to significantly different assessments of the siblings' ancestry.

Still, let's assume my sister did receive a different DNA sequence, which accounted for some variance in the results. That just illustrates the fundamental problem with the tests: too many variables.

Most testing companies compare snippets of a person's DNA to that company's database of DNA markers from people living in various regions of the world. That would work flawlessly if our ancestors had stayed in one place, but they moved around. Once ships and roads were invented, it threw everything off kilter, genetically speaking.

Let's say you had a great-great-great-grandfather from Germany named Otto, who was a playful and libidinous sort. He spends a youthful summer in Italy, where he impregnates a local woman, maybe without even knowing it. He returns home, marries a German woman, and has one son, Hans, your great- great-grandfather. Hans eventually moves to the United States, marries an American woman of Irish descent. They have kids, generations pass, more kids.  You are born and grow up confident of your German-Irish heritage.

But Otto's kid in Italy has been growing his family too, having kids and more kids. By the time you are born, Otto's DNA footprint in Italy could be as big as it is in Germany. So when you take the DNA test, your DNA also matches all these people from Italy. Voilà! Ancestry determines you are likely 20 percent Italian.

"If you really wanted to know where your ancestors lived, say 500 years ago, you'd have to compare your DNA to a database of the DNA of people who lived 500 years ago," said Bolnick.

And that's the bottom line: Today's inhabitants of an area might have a different genetic makeup from its ancestral inhabitants. Just because your DNA matches someone who currently lives in a particular region, that doesn't necessarily mean your ancestors came from that place.

It's a particularly knotty problem for African Americans. More than 11 million Africans were forcibly shipped to the Americas during the slave trade and there were no lists of passengers or records. DNA testing might be the only way many African Americans can discover their African heritage. But some experts say there has been too much mixing of west African groups to correctly identify a country, much less a tribe.

"I can tell African Americans where they are from without a DNA test — anywhere between Senegal and Angola," said Bert Ely, professor of biological sciences at the University of South Carolina.  "These tests are fine at identifying continents but the more specific you get — in trying to identify countries and tribes — the greater the probability of inaccuracies."

But to many African Americans, the tests have real value, even if they can only give an estimate.

"If the test indicates that your DNA matches the Yoruba in Nigeria, that is some real information for some people, something to hold on to," said Charmaine D. Royal, associate professor of African and African American studies at Duke University.

Regardless, questions about accuracy haven't hurt sales. Ancestry has sold nearly 4 million DNA testing kits at $99 a pop. And sales show no sign of slowing down.

"Even if the numbers are often off, they're still fun to talk about at parties," said Marks.

Bob Carden is a freelance writer from Bethesda, Md.