It’s almost the holiday season, which means that you may have already started seeing advertisements to celebrate by ... figuring out that your grandma isn’t really your grandma?

OK, so ... that’s not exactly how DNA test companies advertise. The makers of kits like 23andMe, FamilyTree DNA, MyHeritage and National Geographic DNA talk about helping people figure out their true ancestry, connecting with unknown relatives and learning more about their heritage. In reality, though, the tests have led to more than few deep, dark family secrets being blasted into the open, as well as privacy concerns about the implications of companies owning DNA databases.

So what should you do if a family member suggests getting DNA tests this year? Hopefully our guide can arm you with all the knowledge you’ll ever need on these kits.

Great question. Typically, when you request a kit, you’ll get a package in the mail that requests that you swab your cheek with what looks like a giant Q-tip, or spit in a little collection container. This collects what’s known as buccal material. Then, you send it back to the company, where they perform a test that essentially breaks down all the nucleobases, or letters A,C,T and D, of your DNA.

There are three basic types of tests: autosomal, Y-DNA and mtDNA. Autosomal are the ones you’ll see most often, because they can be given to both men and women and trace back from both your paternal and maternal sides.

As they decode the orders that your letters are in, a process known as sequencing, information is revealed about you – such as where many of your paternal ancestors may have lived, or if you’re predisposed to be lactose intolerant.

Once the testing is over, the companies will put together a little report for you. This will vary depending on the focus of the companies, but most will give you a breakdown of where your ancestors were from, and let you know if you’ve “matched” genetically with anyone else in their system. For instance, if you and your biological parents submitted your info at the same time, it should tell you that you’re a maternal match to your mom.

You might also see you’ve matched with incredibly distant relatives you had no idea existed, such as 10th cousins, but that’s just because the nature of DNA means that humans are more closely linked than we often realize.

That’s one of the big questions with these tests. Are they really accurate? Can a simple cheek swab really tell me who my sixth cousin is? The answer is ... maybe? As with any medical test, some are more accurate than others.

One big thing to remember is that information is compared to other DNA samples within the database. This makes them more accurate for people who have European descent, since that’s the majority of people who have used these kits. Other investigations have revealed that different tests sampling the same DNA have come back wildly different – or, in an extreme case, weren’t able to identify the DNA of a dog.

DNA tests designed to shed light on medical issues are under the spotlight the most, partially because of inaccuracies but also because they can come back as unnecessarily alarming for people who don’t understand the complexity of medical jargon and DNA.

If you’re going to do a DNA test, experts recommend sticking to the biggest brand names who have been on the market the longest, with privacy policies readily available, such as AncestryDNA and 23andMe.

Check out any advice column and you’ll likely find a story from someone navigating the unintended consequences of DNA tests. These kits may have done more to unearth family secrets than any chatterbox aunt or booze-filled holiday party ever has.

But in addition to people finding a long lost half-sibling or learning they were adopted, DNA tests have led to nabbing criminals. Several children, many detailed in a comprehensive New York Times investigation, learned that their biological fathers weren’t donors who had donated their sperm through an anonymous service – they were actually the male fertility doctors of their mothers. It turns out many had been in the illegal habit of artificially inseminating women with their own sperm through treatments such as IVF, even though they told the mothers that they were using sperm of the patients’ choosing, such as from the mothers’ partners or from donor sperm banks. In addition to emotionally scarring the victims, these crimes may have turned away prospective sperm donors who really did simply want to help families struggling to conceive, but who are now distrustful of the system.

In another harrowing case, DNA helped law enforcement officials finally find the notorious Golden State Killer, who killed at least 12 people and raped 45 women in the 1970s and '80s. He also terrorized some of the victims who were alive in subsequent years. Authorities had his DNA on file, but frustratingly it was of little use, since it wasn’t a match to anyone in the system.

But all of a sudden, it was part of a system – not the FBI’s database, but that of genealogy sites. Officials entered his in and looked for a match, and narrowed the suspects down to a small family. Using detective work, they were able to find the former police officer who committed the crimes, and finally nab a serial killer.

Though the prospect of finding killers is appealing, tracking him down using distant family members’ DNA, along with a corporation storing databases full of DNA, has many people worried about the privacy implications of such tests.

Most companies have policies in place that promise they won’t sell or divulge your information to those that could use that info against you, such as health insurance companies. Most also claim they won’t hand anyone’s info over to law enforcement without putting up a fight.

But with the growing number of people voluntarily opting to put their DNA out there, one study estimates that currently, 60% of Americans with European descent could be traced based off these databases, and that by 2020, that number could potentially grow to 90%.

Is that horrible or great? People have different opinions. Proponents argue that with so much DNA from ordinary Americans, medical research will finally have the samples it needs to start to better identify the genetic precursors to diseases, potentially helping us prevent or cure all kinds of diseases going forward. Plus, it helped us get at least one serial killer off the streets.

But advocates for greater privacy are terrified of a future where none of our information is our own anymore. For one, those databases are vulnerable to cyber attack. Plus, many don’t have clear policies about opting in to sharing your data, so many people may be sharing it farther and wider than they thought. Before you click "buy" on a test, think about whether you want your DNA sitting in a database for decades to come.

If issues about privacy and accuracy have you thinking twice about a DNA test, but you still aren’t sure, you might want to ask yourself these questions before you swab your cheek:

• What do you really want to know? Ask yourself what kind of information you’re seeking. Are you curious about your medical history? If you’re not sure you can trust the info you do have, or if you’re adopted, sitting down with a physician, outlining your concerns and getting more detailed and personalized blood and genetic tests might be a better way to get accurate information and recommendations based on that info. If you’re curious about your ancestral heritage, you might want to try asking older family members for stories and documents they may not have shared before to get some info. 
• Are you OK with your info being out there – and is your family? We’ve already seen the kind of information these tests can unearth. Ask yourself if you’re OK with being responsible for secrets to come out, or for some of your family’s DNA to be in a company’s database. Additionally, if you’re under the age of 18, most places won’t let you purchase a kit. You may want to sit down with your family and chat about the kind of information you want to learn and what you would do if you found any surprises before you get a test.
• What will change based on this new info? Simply knowing your DNA structure doesn’t automatically turn you into a different person. Learning you have a trace of Native American ancestry doesn’t mean you suddenly understand the traditions, culture or plight of the Native Americans in the U.S. Finding out you’re distantly related to Albert Einstein won’t turn you into a genius, though both revelations could guide you down new paths of curiosity and learning that you wouldn’t have otherwise gone down. The info might make you look at the world in a different light or be more curious about different people and cultures. But it’s important to always be eager to learn about all kinds of communities and view them empathetically, not just develop that curiosity and empathy once you learn there may be similarities between you.

Of course, the decision to use a DNA test is entirely your own! Hopefully, you can use this information to decide if it’s the right move for you.