"Looking across the whole genome," Fowler said, "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."Coffee is mentioned.
The study is a genome-wide analysis of nearly 1.5 million markers of gene variation, and relies on data from the Framingham Heart Study. The Framingham dataset is the largest the authors are aware of that contains both that level of genetic detail and information on who is friends with whom.
On average, friends are as "related" as fourth cousins or people who share great-great-great grandparents. That translates to about 1 percent of our genes.
Friends are most similar in genes affecting the sense of smell. The opposite holds for genes controlling immunity. That is, friends are relatively more dissimilar in their genetic protection against various diseases.
Oddly enough, married couples are reported to be dissimilar in immunity as well. The reason is not clear. It is suggested that it helps slow down the spread of diseases if parents differ in their ability to fight off different diseases. Perhaps the same mechanism is involved with friends as well.
Side note: The Framingham Heart Study began in 1948 with 5,209 adult subjects. It is now on its third generation of participants