Consumer tests usually examine the Y chromosome (if the person is male), mitochondrial DNA, and a select number of “single nucleotide polymorphisms,” which can be thought of as unique mutations scattered around the genome. In this case, the test examined nearly 765,000 data points from Sen. Warren’s genome.
But these tests have limitations. For instance, the U.S. National Library of Medicine writes that “ethnicity estimates may not be consistent from one provider to another.” That’s why, as Science News says, DNA tests are just a starting point for genealogy. To really find out who your ancestors were, you must examine “marriage certificates, military rolls, census records, immigration documents, old photographs and other records.”
Anything less than that, and — as Ross Pomeroy at RealClearScience wrote — you’re basically just reading a horoscope.