Tracing Your Ancestry
Advances in DNA testing are allowing people to uncover information about their genetic ancestry and find out where some of their ancestors came from. As an African American, I don’t know where my African ancestors originated from. The only geographic location I can point to as my ancestral home is Tennessee. So I’m fascinated by the potential knowledge I could gain from this new generation of tests for genetic ancestry.
But before I fork over more than $200 for such a test, the skeptic in me needs some answers. What can a DNA test really tell me about where I come from? How do these tests work? And can they be wrong?
Companies that offer genetic testing services for finding out about ancestry use several different testing methods. Lineage-based approaches analyze DNA on the Y chromosome, which is passed down almost unchanged from fathers to sons, or else analyze mitochondrial DNA, which is passed down nearly unchanged from mothers to their children. Small genetic changes in the Y chromosome occur as this information is passed from successive fathers to sons. These changes, if they persist, become markers of descent. Likewise, as mitochondrial DNA is passed down, slight mutations occur, and if these mutations persist, they also become genetic markers that can help distinguish one matrilineal line from another.
Taking these tests is straightforward. A person swipes the inside of his or her cheek for a saliva sample, which is sent to a lab. There, the DNA is extracted, amplified, and analyzed. It is then compared to and matched with DNA samples from a reference database of haplotypes – a set of closely linked genes or DNA polymorphisms – that have been identified in specific populations. If a person’s DNA sequences match certain sequences in the database, the information can be used to determine the populations with which that person shares maternal or paternal ancestry.
“Lineage testing can trace your ancestry back to real existing people who carried that particular DNA type throughout prehistory until today,” explains Peter Forster, a geneticist at the University of Cambridge in the U.K. and cofounder of Roots for Real, a company in Cambridge that uses mitochondrial DNA tests to determine maternal ancestry.
But these methods have a drawback. They account for only a small portion of a person’s ancestry. Mitochondrial testing traces a person’s mother, maternal grandmother, maternal great grandmother, and so on. Similarly, Y chromosome testing traces only one line of a person’s male ancestry, starting with a man’s father, his paternal grandfather, paternal great grandfather, and so forth.
Another strategy for ancestry tracking is admixture testing. This kind of test focuses on the 22 pairs of nonsex chromosomes in every cell. Since one of the chromosomes has been inherited from the person’s mother and one from the father, they contain recombined segments of DNA from all of a person’s ancestors. The test compares an individual’s DNA with specific sequences of DNA that are more prevalent in people from one area of the world than from another area. Admixture testing can determine which of the major bio-geographical population groups a person belongs to – sub-Saharan African, European, East Asian, or Native American. The test results are given as a percentage breakdown.
These tests “can tell you something about lots of people, but they are not exhaustive,” says Mark Shriver, associate professor of biological anthropology at Pennsylvania State University and a consultant with DNA Print Genomics, a Sarasota, FL, company that provides genetic tracing services. Some major groups, including South and Central Asian ones, are not as well represented in the databases, Shriver says. In addition, European groups need to be broken down further and Africa is even more understudied. “We are still limited by the databases,” says Jason Eshleman, senior research scientist at the company. “The world is a big place with a lot of people and when you split it into four populations, you find out there are a lot of populations we don’t know much about.”
In both lineage and admixture testing, the larger the databases used to compare with a client’s DNA, the more accurate the results are likely to be. Even so, large databases may not cover everyone. For example, if a client has a close database match with a person living in western Africa, does this mean that region is the most likely ancestral origin of his or her maternal or paternal lineage – or could that person be a closer genetic match to people from elsewhere, but who’re not yet included in any database?
“We take pride in curating the largest global and proofread academic database offered by any service, more than 35,000 individuals,” Forster explains. “But even so, if we take a sample of, say, 100 Sicilians, the database will indeed map about 95 percent of them to Europe, but it will also identify the other five percent as being nonEuropean.” This could be an error or it could reflect prehistoric migrations in and out of Sicily. “Clearly, the better the sampling of a continent, the lower this kind of error rate becomes, and improved sampling is something we are doing continuously as part of our research,” says Forster.
With admixture testing, if results indicate that someone is, for instance, five percent East Asian, it could be true or it could be an error. If that person has other evidence, however, that he or she has a grandparent of East Asian descent, it makes a stronger case for the results being accurate. “These tests should be interpreted in light of other information,” Shriver says. “You have to take the totality of information.”
Beyond questions about a test’s accuracy there are issues about how such findings might be used. Some social scientists point out that a person’s ethnic identity is far more than genetics. While using DNA testing to learn about your genetic family tree is one thing, hanging one’s entire identity on a genetic test is problematic, says Paul Brodwin, associate professor in the department of anthropology at the University of Wisconsin-Milwaukee. “The actual knowledge is disproportionate to what people do with it,” he says. “What people do with this knowledge is weave a story of their identity. Some seize upon a tiny piece of evidence to say, ‘ah-ha, now I know where I come from and who I am’.”
Standard genealogical sleuthing – combing through historical documents, reading books, talking to older relatives, and remembering oral stories passed down – may give a person more information about their ancestry than a DNA test, says Brodwin.
So what is the value to me of finding out where in Africa some of my ancestors may have come from? Is it worth the $200 or more that tests might cost? Sure, because it’s important for me to know anything about my African ancestors. But it’s also important not to get carried away with the information. If a test suggests that I have lineage in Ghana, I’m not going to all of sudden start calling myself a Ghanaian. Nevertheless, I will have a small bit of information about some of my ancestors. And that’s well worth knowing.
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