Eye on DNA has an interview with a "genetic genealogist." He's a little more enthusiastic about the informative value of these tests than I am, but the interview itself is pretty informative.
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You can find out. Blaine Bettinger, the Genetic Genealogist has a fun little quiz.
Blaine Bettinger at the Genetic Genealogist has an extensive and thoughtful critique of the American Society of Human Genetics' recently released
Information Processing: Bell and GHZ: spooky action at a distance