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Last fall, Sgt. Timothy Gall, an Army medic stationed at Fort Belvoir, sought clues to the multiple sclerosis and heart disease that ran in his family by looking into his DNA. All it took was some spit and about a thousand bucks.

He didn't go to a doctor. Instead, Gall, 30, joined the growing number of consumers ordering scans of their DNA directly from private companies. A handful of companies such as 23andMe, Navigenics and Decode Genetics offer customers a personal peek at their genetic code, finding variations linked to certain traits, diseases and drug sensitivities -- a process known as genotyping. As the cost of genetic scanning has dropped and the pace of genetic discovery quickened, these companies began springing up about three years ago. Now, as they're attracting more and more customers, they're also drawing more scrutiny.

Once Gall decided to try it, he persuaded his father to be genotyped, too. As a quality check, both men mailed saliva samples to two testing firms: 23andMe, based in California's Silicon Valley, and Decode, of Reykjavik, Iceland. "I figured I'd...