Humans have some 6 billion bits of genetic code in their DNA. Among these are about 20 million gene variations called single nucleotide polymorphisms (SNPs, pronounced snips). SNPs are chiefly responsible for making one human being different from all others. In the 1990s, scientists began looking for groups of people with the same SNPs who also have, say, colon cancer. Find enough who have both the same SNP pattern and colon cancer, and they might identify a genetic signpost for the disease.

Some of these “association” studies have led to breakthroughs. Already, thousands of women each year take tests for mutations in BRCA1 and BRCA2, two genes associated with a highly hereditary form of breast cancer. Researchers have also pinpointed a few monogenic conditions, in which a single genetic variation is directly connected to a particular ailment, such as Huntington’s disease. If you have the Huntington’s SNP, you’ll almost certainly contract the disease.

But it wasn’t until a few years ago that researchers began to unlock the vaults of information stored in our genes. In 2003 scientists completed the Human Genome Project, which mapped our common genomic sequence, determining the order of four component chemicals in each of our DNA’s 3 billion base pairs and identifying all of our approximately 20,000 genes. Two years later they finished the first phase of the HapMap Project, an effort to catalogue common patterns of SNPs that initially pinpointed 1 million variations. Phase 2, completed in October 2007, added 2 million more SNPs. Not surprisingly, the connections between genes and disease are turning out to be extremely complex, with the interplay between genes and prevalent ailments still undeciphered. Diet, lifestyle, environmental factors and stress also come into play, and the presence of certain SNPs, as revealed by some of today’s genomic tests, is just one of many possible contributing factors.

Amid such uncertainty, David Altshuler, a geneticist and physician at the Massachusetts General Hospital and Harvard Medical School, has turned down offers from DNA vendors to scan his genome. “Right now, there isn’t enough known to make the offerings of these companies clinically useful,” Altshuler says.

Consider type 2 diabetes. If Altshuler’s patients want to gauge their chances of developing the disease, they could be tested for the TCF7L2 gene, a mutation carried by more than a third of the population that increases the odds of the disease’s onset by more than 45%. But environmental factors and diet also play a huge role (obese people are much more likely to become diabetic), and many unmapped genes might be even more closely associated with the disease. Today, Altshuler says, reviewing a patient’s family history reveals more than a test showing the presence of TCF7L2—“by a long shot.”

The science, in other words, currently lags the hype. “If there’s one take-home message in what scientists have learned about genomics during the past 10 years, it’s that this is a lot more complex than we ever thought,” says Jesse Reynolds, director of biotechnology accountability at the Center for Genetics and Society in Oakland.

Some think this need not be a deterrent. Elissa Levin, director of genetic counseling at Navigenics, says that customers have shown an ability to grasp that genetics are just one part of the equation. “There’s a shift away from the idea that genes are our destiny,” she says. “What I keep hearing is that our service is really a heads-up. People actually feel empowered.”

What’s more, there continue to be breakthroughs in linking genes to disease. In a prostate cancer study published last January in The New England Journal of Medicine, researchers at Wake Forest University School of Medicine in Winston-Salem, N.C., and other institutions reported that a grouping of common, minor variations in five areas of DNA can go far in forecasting the risk of getting the disease. In a study of nearly 5,000 men, the researchers found that 90% had one or more of the SNPs, and more than half had two or more. The cancer risk grew as the number of variants increased. Those who had at least four of the variants were five times more likely to get prostate cancer than those who had none. Add in a family history of prostate cancer, and risk of the disease skyrocketed to nine times that of men with no genetic risk factors.