Most DNA tests have traditionally relied on only two small parts of the genome: the Y-chromosome, which is passed down almost unchanged from father to son, and mitochondria, which mothers pass faithfully to their offspring. Because these stretches of DNA remain relatively consistent from one generation to the next, they are particularly useful for testing direct-line paternal and maternal ancestry, respectively; however, they essentially ignore the bulk of someone’s DNA ancestry and cannot detect genetic signatures that cross gender lines.
But the test that 23andMe offers is different. Available commercially for only a few years, it measures close to a million single “letters” of DNA smattered across the whole genome to reveal ancestral origins of, and risk factors for, almost 100 diseases. And with the $99 sale price the company was charging for the test in December — much less than similar tests — more people have been rushing to use the service and are getting surprising results.
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