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Assuming the tests are done accurately, some discrepancies can still arise from differences in the companies’ DNA databases. Almost every DTC genetic test does not sequence your entire genome, but instead looks at positions in your DNA that are known to be of interest. When I was tested by 23andMe, they proclaimed I do not carry a version of a gene that is associated strongly with red hair. Another ancestry company said I did. This merely reflects the fact one company was looking at different variants of the gene that code for ginger hair.
If we assume the data generated is accurate, then the second question that arises is on the interpretation. And this is where it gets murky. Many of the positions of interest in your DNA are determined by experiments known as Genome Wide Association Studies, or GWAS (pronounced gee-woz).
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Each company offering these services uses its own proprietary database of DNA samples called ancestry informative markers (AIMs) from current populations in Asia, Africa, Europe, and the Americas. From within those databases, they each select for a certain number of alleles—one member of a pair of genes located at a specific position on a specific chromosome—and in these spots, use the genetic variations known as single nucleotide polymorphisms (SNPs) as the basis for evaluating individuals. The markers—SNPs—are chosen because they have different frequencies across different geographical populations.
They compare SNPs with those most frequently associated with different populations in their reference database. The results are in no way definitive; instead each company uses common genetic variations as the basis for saying the probability is that 50 percent of your DNA is, for example, from North Europe and 30 percent is from Asia, based on how it compares to the information in its database. However, if you send DNA to a second company, you might get different results, because it has a different database. Studies that have compared ancestry databases have found poorer concordance with Hispanic, East Asian, and South Asian descent.
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There’s a big chunk of data—actually the majority—that these genetics-testing services don’t use. Your DNA contains millions of SNPs, but these tests are selectively looking at certain genetic variations and use between 100 to 300 AIMs, which account for a small part of the SNPs that differentiate the human family. So even if a test says you are 50 percent European, really it can only report that half of those SNPs of your DNA looks to be European.
The results are further skewed by the fact that certain ancestry information markers used by any particular test may come from only your paternal line (Y chromosome) or your maternal line (mitochondrial DNA). Tests using these markers are less accurate.
Finally, these testing services use DNA from modern populations in these regions to draw conclusions about people who lived in those areas hundreds or thousands of years ago. It’s a big leap to assume that the particular SNPs used by the tests have remained constant for all that time.
