If you’ve ever wanted to know just exactly how much DNA you share with your ridiculously tall brother or doppelganger best friend, you’ll soon be able to find out. 23andMe, a personal genomics startup in Mountain View, CA, is about to unveil a new social-networking service that allows customers to compare their DNA. The company hopes that the new offering will encourage consumers to get DNA testing, potentially creating a novel research resource in the process.

“I think the idea of social networking has untapped potential,” says George Church, a pioneer in genomics at Harvard Medical School in Boston and a member of 23andMe’s scientific advisory board. “The idea has precedence in PatientsLikeMe, people who have been enabled to find one another by their disease. Here, people can find each other by their alleles [or genetic variations].”

23andMe is one of a number of companies that have sprung up in the past year to offer genome-wide DNA testing directly to consumers. People who order the $999 kit send in a sample of spit and, in return, receive an analysis of nearly 600,000 genetic variations linked to disease and other factors, such as ancestry, height, and eye color.

These fledgling companies have garnered both hype and criticism, with most of the controversy centered on the medical applications. Customers can learn their genetic risk, compared with the general population, of myriad diseases, including Alzheimer’s, diabetes, macular degeneration, and cancer. But many scientists and physicians say that it’s unclear whether the average user can truly comprehend this information, and whether knowing her genetic risk will actually improve her health.

Like its competitors, 23andMe offers information about an individual’s disease risk. But it has also opted to emphasize more entertaining approaches to personal genomics, including using colorful visualization tools to look at a subject’s ancestry and compare it with that of celebrities from Jesse James to Benjamin Franklin and Bono. Now, to capitalize on the boom in social networking, the company will launch a genome-sharing tool that allows people to compare their genome with those of family members, friends, and even strangers who have offered up their DNA data. “It seems like the first natural curiosity people have is, where do I come from? What are my roots?” says Linda Avey, who cofounded 23andMe with Anne Wojcicki. “The next natural [question] is, how do I compare to other people?”

Avey and Wojcicki hope that, as with traditional forms of social networking, word of the new tool will ripple through family and friends, bringing in new customers. Avey, who has shared her genome with many users, says that she has introduced people based on their haplogroups: genetic groupings representing major migration branches of the human evolutionary tree. Perhaps haplogroup inquiries will become the genomic alternative to the classic pickup line “What’s your sign?” Or maybe genome-savvy geeks will create haplogroup-specific MySpace pages.

“This may serve a social need that seems to be out there,” says Mildred Cho, a biomedical ethicist at Stanford University. “It seems to be an extension of people forming groups based on a particular disease or genealogy. But there’s also some danger in overmagnifying the significance of those similarities. There is some skepticism about how meaningful this genetic information really is.”

At this point, the genome-sharing tool is mostly fun. But Avey and Wojcicki say that their ultimate goal is to create a genomic database large enough to reveal scientifically and medically relevant information about its users. Those who order the genome kit can indicate whether they are willing to participate in future research studies. Those who opt in fill out surveys detailing their medical histories and specifics about particular conditions. The result will be a database of both genomic and phenotypic information, similar to that collected by government and academic institutions for research. But the difference will be that participants know (and in fact paid for) the details of their genome.

“It’s an intriguing idea to have a space for public participation in developing science,” says Greg Feero, chief of genomic health care at the National Human Genome Research Center, in Bethesda, MD. “The more we can engage the public in thinking about genomics, the better off we’ll be in the long run in developing this new branch of medicine.”

However, Feero says, the power of the database will lie in the accuracy of both the genetic analysis and the medical and personal information provided by customers. “If 23andMe is going to do this well, they’ll need to be incredibly careful in how they label an individual as having a particular trait,” he says.

Feero says that it’s also unclear how companies like 23andMe will use the data that customers put into its bank. “Will it be used only for public research endeavors, or also private product development?” he asks. Avey has said that 23andMe will not sell genetic data to pharmaceutical companies, but it may sell access to the company’s pool of willing research participants or other fruits of its database.

While 23andMe is gauging interest in the enterprise from the research community, the company’s founders also envision spurring a sort of grassroots research effort that mirrors the rising influence of patient-advocacy groups, such as those that have organized new research projects about autism and Parkinson’s disease. If the trend attracts large enough numbers, people with particular diseases could come together to search their genomes for similarities. Or those who escaped a particular condition despite a high genetic risk could provide insight into lifestyle and other genetic factors that were protective.

“This might be better done by the research community than a social-networking community,” says Church. “But it might be well started in this playful environment.”

Avey gives the controversial example of the link between vaccines and autism. Large-scale studies have found no association between the two–a finding accepted by most of the research community–but a subset of parents remain convinced that their child’s disorder was triggered by vaccinations. This group could potentially search for genetic resemblance among their children, perhaps in genes involved in drug metabolism. “If we can give them a platform to sign up children and do their own study, we want to facilitate that,” says Avey.

Of course, success of a community-driven approach to genetic testing requires huge numbers of parents to sign up for 23andMe’s service. For the autism example, a member of 23andMe’s scientific advisory board estimates that 20,000 children with autism would need to be signed up for the company’s service to generate a large enough database for research. 23andMe declined to release current sales figures.

The idea of simultaneously paying for a service and participating in research is not unprecedented.”The National Genograpic Project gathered half a million participants in a fairly short period of time,” says Church, who is overseeing a similar, nonprofit effort to gather and analyze genetic and medical data. “If you start getting serious numbers of volunteers, it could be awesome.”