Christie Aschwanden / Special to The Washington Post

What does your DNA really reveal about your health?

It sounded enticing: For just $99, I could spit into a tube, mail it off to a company called 23andMe, and, six to eight weeks later, I’d receive a report explaining what my DNA reveals about my risk for 120 diseases.

“Knowing how your genes may impact your health can help you plan for the future and personalize your health care with your doctor,” the 23andMe website declares. “You’ll have access to navigational tools that enable you to explore your genome and discover a whole new world of you.”

Based in Mountain View, Calif., 23andMe is just one of several companies that sell genetic tests directly to consumers. GenePlanet, a company based in Slovenia, markets a test that costs 399 euros, or about $518, and claims to tell you the best diet for your genotype, and Genetic Testing Laboratories promises to disclose “your predisposition for cardiovascular conditions, cancers, immune system (issues), general health issues and much more” through a test that sells for $285.

While these claims may seem outlandish, most have at least a snippet of real science behind them. The tests look at a type of genetic variation called a single nucleotide polymorphism, or SNP (pronounced “snip”). Your DNA is made up of four kinds of nucleotides — adenine, cytosine, guanine and thymine (A, C, G and T) — and a SNP is a single alteration to one of these nucleotides that’s found in at least 1 percent of the population. For example, if most people have the sequence CAGGCTG at one site on the genome, those with a certain SNP might have TAGGCTG.

The notion that these tests can help you calculate your risk of disease are based on studies that compare SNPs in people with a particular condition to the SNPs of those without the disease. If a particular SNP is more common among people who have the condition or trait, this suggests that the condition and the variation may be related, but it’s not proof of a cause-and-effect relationship, says David Kaufman, director of research and statistics at the Genetics and Public Policy Center at Johns Hopkins University.

“It doesn’t mean that if you have the SNP, you’re going to get the disease and if you don’t, you’re not,” he says. Even if you do have a SNP associated with a disease, your increased risk is usually small — on the order of 10 or 20 percent more than it would be without the SNP.

While it’s clear that many diseases do have a genetic component, very few medical conditions come down to a single gene or to genetics alone, says Jeffrey Murray, a geneticist at the University of Iowa School of Medicine and president of the American Society of Human Genetics.

Identical twins, who share the same DNA, rarely end up with exactly the same medical conditions, and that tells us that genes alone cannot predict a person’s medical future, Murray says. “What you look like is almost 100 percent genetic, but what you’re going to get isn’t. There are lots of other things — random chance, environmental exposure and all kinds of stuff that we can’t control.”

If that’s the case, why get tested?

“You’re more prepared for things. It enables you to prioritize your health care activities,” says Joanna Mountain, 23andMe’s senior director of research. “For instance, if you know you’re at higher-than-average risk for an eye disease called macular degeneration, it might prompt you to get your eyes examined more regularly.”