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'Play Defense with the Facts'

Like other tests that delve into medical history, personal genome scans like those from 23andMe can reveal unexpected news. Slate tells the story of "Jackie" and her brother "Alex." (Slate changed their names.) Jackie, who works in a biomedical research lab, decided to get the 23andMe scan to learn about her disease risk, but she also found out that Alex, who also was tested, was her half-brother, and the man she thought was her father was not.

Previously, doctors could determine such instances of nonpaternity — the exact rate of which is unknown, but Slate says is thought to be about 2 percent or 3 percent for overall population — through blood type testing.

Such information can now, though, be gained through not only blood typing or more focused, specific paternity tests, but through other not-so-specific tests that people might be pursuing for other reasons.

23andMe, Slate notes, has two layers of consent to go through before it will show familial relationships. "This quirky system shows the difficulties that arise in managing genomic data," Slate adds. "It used to be that people chose to learn about themselves or not, and doctors helped determine which bits of information were appropriate for each of us to know. Now we're heading for a place where secrets flow more freely, where wise consumers must play defense with the facts."