The names for those ancestry groups, Stoneking said, come from a mix of self-reports (many people can describe their immediate background pretty well) and independent research. So, if your 23andMe test says you're 29 percent British, it's because 29 percent of the pieces of your DNA were most likely to have come from a group that 23andMe's reference library has labeled "British." It compares it against British it compares it against West African it goes through the entire list, and it spits out a probability for. "It takes little pieces, and for each piece, it compares it against the reference data set. "The way that the algorithm works, it takes an entire genome and chunks it up," Smith said. The richest and most detailed ancestry information, however, comes from comparing everything else - the 22 non-sex chromosomes - against the massive libraries. Maternal ancestry, similarly, can be found in mitochondrial DNA, which mothers pass to all of their children. Researchers can track paternal ancestry by looking at the Y chromosome, which fathers pass to their male children. If a genetic anthropologist has a DNA sample and a very large library of other samples to compare it against, that anthropologist can quickly figure out which groups in the library that DNA is most closely related to, Stoneking said. And researchers still use these methods to track the movements and intermixing of human populations from the deep past to recent history. He used a version of these methods in his pioneering work tracing the common ancestor of all living humans, a woman referred to as " Mitochondrial Eve" who lived about 200,000 years ago. "These are techniques that scientists have known about for a very long time," Stoneking told Live Science. Instead, according to geneticist Mark Stoneking, group leader of the Max Planck Institute for Evolutionary Anthropology in Germany, they're really good at spotting patterns. But it's not that their computers speak some secret language. These companies keep the details of their algorithms somewhat secret. But the algorithms can pull meaning out of the strings of letters, Smith said. There's no string of letters that means "Swiss" or "Nigerian," for example. This string of letters would be incomprehensible to you and, on their own, just as incomprehensible to the biologists and engineers who study them.
Those are the labels given to the four nucleobases of DNA, the letters with which genes are written. Once the DNA in a spit sample has been digitized, it looks like a long string of C's, G's, T's and A's. Smith told Live Science that 23andMe uses a number of algorithms to arrive at these results. But how do these services actually work to determine someone's ancestry?
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