A fascinating study shows that folks who have a special kind of dopamine receptor tend to become friends. In this case, it's DRD2, a gene sequence involved in producing a dopamine receptor, which is a marker for alcoholism. (Dopamine is the brain chemical of reward-seeking and reward; it's involved in addiction to substances, thrill-seeking and romance.)
In the article in Medical News Today, James H. Fowler, of the division of medical genetics at the University of California, San Diego explains:
"We live in a sea of genes. What happens to us may not depend only on our genes but on the genes of our friends. This might be the first step towards understanding the biology of 'chemistry,' the feeling you have of you whether you like or dislike a person [almost immediately]. We might choose friends not [only] because of social features we consciously notice but because of biological and even genetic features that we unconsciously notice. "
He talks about the relationship between genes and behavior, and how this may cause us to bond with people like us. And he also discusses his findings that in some cases, people with a genetic marker for openness tend to flock with their opposites.