A person's friends tend to share certain genes in common with each other — but not always with the individual, a new study suggests.

"People’s friends may not only have similar traits, but actually resemble each other on a genotypic level," said the study led by James Fowler, a geneticist at the University of California at San Diego.

The findings were published Friday in the Proceedings of the National Academy of Sciences.

Researchers noticed two distinct patterns within social networks when it came to the genes DRD2, which has been linked to alcoholism, and CYP2AP, which is linked with the character trait of openness.

In the case of DRD2, people with the marker tend to make friends with those who also have that marker. People without it tend to make friends with other DRD2-negative individuals.

In the case of CYP2A6, the person who has the gene tends to be the hub of a social network made up of people who don't have it and instead share the opposite genotype.

Four other genes examined by the researchers did not show such patterns among groups of friends.

The analysis found that this gene clustering within social networks was apparent even when the researchers took into account the fact that people are more likely to make friends with people who live near them.

The findings suggest that studies linking certain traits to genes may be biased in ways that were not previously anticipated.

"For example, a person with a genotype that makes her susceptible to alcoholism may be directly influenced to drink," the study said. "However, she may also be indirectly influenced to drink because she chooses friends with the same genotype … who are more likely to make alcohol available to her."

The study suggests that when trying to link certain characteristics with a particular gene, researchers should test not only the subject's genes but also those of his or her friends.

The researchers conducted the study by comparing six available genotypes from the U.S. National Longitudinal Study of Adolescent Health. They mapped the friendship networks of teens involved in the study.

After finding that DRD2 and CYP2A6 showed common patterns within people's social networks, they looked at data from a different study, the Framingham Heart Study, and found similar patterns.