DNA tests have become a popular gift for all generations.

They're easy to use — just send 23andMe, Ancestry, or one of their competitors your spit, and in exchange you're promised a detailed breakdown of your heritage. For a little extra cash, you can learn about your genetic health risks, too.

23andMe and Ancestry are privately held companies that don't disclose sales numbers. But ads for them are all over the internet these days. In 2017, Ancestry claimed it sold 1.6 million kits (which regularly start at $99) in the four days leading up to Cyber Monday.

The appeal is obvious. It's fascinating to learn where your family is from. And when it comes to your health, the more you know, the better, right? Well, before you send in your saliva, you might want to know a few things.

You're giving them your family's DNA, too

Obviously, you share DNA with your relatives. That's why ancestry tests work in the first place.

But think for a second what that means. If you provide genetic information about yourself, you're also providing information about them — whether they consent to a DNA test or not. Back in April, the suspected "Golden State Killer" was caught because his distant relative used a service called GEDmatch.

Catching a murderer = a good thing. And yet, the incident shows how people can be tracked with DNA tests even if they don't take one, a scary concept if you're living under a repressive regime, or you're just freaked out in general about your privacy.

It's 23andMe's policy to "resist law enforcement inquiries to protect customer privacy," the company said when the suspect was apprehended. And Ancestry said it will "not share any information with law enforcement unless compelled to by valid legal process."

So, basically, you'll have to trust them. And there are other services that don't promise the same level of privacy.

Your DNA could be sent to a corporation

Your genetic information is valuable. Specifically, to pharmaceutical companies. Over the summer, 23andMe received a $300 million investment from GlaxoSmithKline. As part of the agreement, the pharmaceutical giant will get access to anonymized data from 23andMe's DNA kits.

That could be very useful for developing drugs. But, again, you've got to trust several companies to keep your data private — which, these days, can be hard to do. If you don't want your genetic profile passed around, both 23andMe and Ancestry let you opt out of sharing your DNA for research purposes.

White people. Image: Shutterstock / AboutLife

Ancestry tests work great ... for white people

Companies use different algorithms based on different sets of data. Most of that data comes from people of recent European ancestry.

The problem, obviously, is that a lot of people don't have grandparents or great-great-great-grandparents from England or Italy or Denmark. Most people on Earth, actually! That means if you're from, say, Asia or Africa, you might not get as detailed a profile as you'd like.

My mother, who was born in the Philippines, actually got an update from 23andMe with new information about her heritage. Her history didn't change. But as the company gets more DNA kits from people of Asian descent, the algorithm churns out modified results. Which is great ... but that does mean right now, if you're not white, you might have to wait a bit longer for more accurate results.

They're not a replacement for medical tests

Earlier this year, 23andMe won approval from the U.S. Food and Drug Administration to market its test for genetic mutations that could signal increased risk for breast cancer.

The company can also look for markers for celiac disease, Parkinson's, and late-onset Alzheimer's disease. Tests can let you know whether you're a carrier (meaning you may not have the condition, but you could pass on a genetic variant for the disease to your children) for cystic fibrosis, sickle cell anemia, and other conditions.

These tests are not a panacea — they're a starting point.

Having that information can help you prepare for the future. But the tests don't provide a complete picture. As NPR noted in June, 23andME only tests for three mutations in three genes for breast cancer. There are thousands of other mutations that increase risk, as well as environmental factors.

The risk is that people will think they're in the clear, and not bother with mammograms or other tests afterward. These tests are not a panacea — they're a starting point. That's something these companies need to make clear.

You might discover something you don't want to know

Sometimes, ignorance is bliss. There was that guy who found out his teenage daughter wasn't his and filed for divorce. Or the woman who was shocked and dismayed to find out she wasn't raised by her biological father.

In fact, as Bloomberg recently reported, these kinds of surprises happen so often, customer service reps at 23andMe get months of training to learn how to comfort distraught callers. (It's so stressful, the company's head of customer care said she "encourages representatives to go for a walk after an intense call, or cracks open a bottle of wine to help them decompress.")

If none of these factors scare you, then have fun. These tests are almost always on sale. Just know what you're getting into.