You and your friends are surprisingly genetically similar, according to a study appearing in PNAS. The similarity isn't immense -- about 1%, roughly the same as between fourth cousins -- but it's significant enough to be detectedand to have evolutionary implications.

To measure the genetic similarity between friends, Nicholas Christakis and James Fowler used data from the Framingham Heart Study, a long-term cardiovascular study in which participants are examined every two years. According to Christakis and Fowler, the Framingham dataset is the only one "of any significant size" with both information on social ties and detailed genetic data, making it uniquely suited for analysis the correlation of genotype and friendship. They examined around 450,000 polymorphisms in the genomes of nearly 2,000 people and correlated their similarity within the roughly 1,400 pairs of friends within the group. Next, they compared the correlation between friends with the correlation between the (more than a million) non-friendship pairs in the same group. "Looking across the whole genome," said Fowler, "we find that, on average, we are genetically similar to our friends. We have more DNA in common with the people we pick as friends than we do with strangers in the same population."

The tendency for friends to have similar genomes might result from people befriending those of similar ancestry, but the duo are confident that isn't the case. Not only did they control for the effect of population structure and ancestry, but the Framingham data is relatively homogeneous in terms of ethnicity, consisting largely of people of European descent. In addition to the overall genetic similarity between friends, there were also a few sites in the genome which usually differed between friends, a pattern that's harder to attribute to factors like demographics.

Christakis and Fowler didn't stop at simply measuring genetic similarity; they also wanted to figure out what kind of genes were contributing to the similarity. By analyzing 1.5 million polymorphisms across the genome, they identified sets of genes that were likely to be similar or different between friends. Topping the list for common genes were those related to our sense of smell. One explanation for this is that people with similar olfactory genes will smell things in the same way and so be drawn to (or repulsed by) similar environments. Some people are more sensitive to smells than others, and it seems that may have a significant impact on our social connections.

The opposite pattern held for genes related to immunity -- friends tended to be less similar at those parts of the genome. It's not hard to imagine why. After all, if you're succumbing to an unpleasant bug, it would be great to have friends who are resistant to it and could help nurse you back to health. On the other hand, it's not entirely clear how we pick friends with differences in their immune-related genes. There's some evidence that humans use body odour to evaluate aspects of a potential mate's immune response, and a similar process might be at work here. Whatever the case, it seems that one aspect of who we choose to befriend is their genetic make-up, whether we suss it out based on environmental clues, like a similar sensitivity to smells or temperatures, or get swayed by intrinsic cues such as body odour.

One of the most interesting findings of the study is that the genotypes which are shared by friends are more likely to have experienced positive selection than other genotypes. In other words, the similarities between friends are being selected for and impacting human evolution. Our evolutionary environment isn't just made up of physical and ecological factors, but also includes the social environment. The ability to collaborate with others who aren't our kin offers significant advantages, and the dynamics and benefits of those collaborations may depend on a certain genetic compatibility between those involved. "The paper also lends support to the view of human beings as 'metagenomic,'" said Christakis, "not only with respect to the microbes within us but also to the people who surround us. It seems that our fitness depends not only on our own genetic constitutions, but also on the genetic constitutions of our friends." We may have evolved to seek out genetically similar people as friends because forming social groups with those people enabled us to reap the largest rewards for cooperation. As James Fowler put it, "The first mutant to speak needed someone else to speak to. The ability is useless if there's no one who shares it. These types of traits in people are a kind of social network effect."

https://www.nature.com/scitable/blog/accumulating-glitches/friends_are_genetically_similar/