It sounds like such a fun holiday gift idea: a DNA test that can tell your sister-in-law whether she really has Native American ancestors, or one that promises to craft your friend a perfect diet based on his genes.

Home DNA tests are likely a big seller for the next few weeks, but privacy experts say consumers should be cautious, and New York Sen. Chuck Schumer said this past weekend that he was asking the Federal Trade Commission to “take a serious look at this relatively new kind of service and ensure that these companies can have clear, fair privacy policies.”

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Even if you do read the whole agreement, which can go on for pages, you may not understand what you’re giving the company permission to do, said Hank Greely, director of the Center for Law and the Biosciences at Stanford School of Medicine.

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“There is no legal limit on what they could do other than the agreement that you enter into with them which they may or may not choose to follow,” Greely added. “If they don’t follow it, the chance you would ever find out is very, very low.”

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“Even if you just send your DNA in for genealogical work, what those companies typically run is a SNP test hundreds of thousands of markers, even though they may be only looking for a couple of hundred markers,” Greely said. A SNP (pronounced “snip”) is a single nucleotide polymorphism, a single-letter difference in the genetic code that may cause disease or that may lead back to your great-great grandfather.

“That analysis shows things about your health that the company never told you because that is not the business they are in,” he said. “They are in the genealogy business.”

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“For a non-trivial percentage of us, there really are scary things in our genomes,” he said.

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“Especially if it is coupled with health information, you can say this is a 39-year-old woman from Westchester County who is five feet, seven inches tall, who has blue eyes and has cystic fibrosis — it wouldn’t be that hard for somebody to find you,” Greely said.

“Now would anybody try? I don’t know. If you are a member of the royal family or a celebrity of some sort, I suspect people would. Is there a snoopy relative? Is there somebody just curious about you?”

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“You cannot promise people absolute confidentiality,” Greely said. “The other side of it is that it’s possible that somebody will hack into a company database that does contain your information. My financial information has been hacked three times in two years. All that stuff is out there.”

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