Companies that make take-home genetic tests — like 23andMe — are spending a lot of money on advertising to make their product seem like the perfect holiday gift.

But medical ethicist Arthur Caplan says opening that package under the tree could be like opening Pandora's Box.

One concern is privacy: Forbes reported in February that many popular genetic testing companies sell their data to outside companies. 23andMe, for example, has sold information to at least 13 pharmaceutical firms, the article says.

"You don't know what they're going to do with it in terms of resale or giving it to another company. You don't know how good their labs are or how good their privacy protection is," Caplan said.

"It's like saying: 'Here's my credit card, I'd like to learn more about the amazing world of finance. Let's all bond together as we compare our credit card scores,'" he continued.

There's also the possibility that the test may reveal information you may not necessarily want to know. 

"There's all kinds of things that could come in on these tests," Caplan said. "You could find out about paternity or incest or all kinds of things I don't think I want to be sharing around the old Yule Log." 

There have also been cases when people have found out from genetic tests that they have genes associated with risk of disease — like the BRCA 1 and 2 genes, which are associated with breast and ovarian cancer — and have been  denied for life insurance as a result.

"You find out you have a pre-existing condition [and] you can't get life insurance, you can't get disability insurance," Caplan said.

Click the audio player above to hear more from Art Caplan.