It appears that when it comes to choosing our friends, much of our choice depends on genes. We are more genetically similar to our friends than we are to strangers.

The theme tune to Friends, a 90s American sitcom about a bunch of best mates. Two of the characters were related, but if they’ve been real life friends, the rest of them could have been as biologically close as cousins. Researchers from the University of California have analyzed gene variation in around 2000 people, comparing pairs of unrelated friends with pairs of unrelated strangers. They found that the friends had about 0.1% more DNA in common on average than strangers did. It’s a small fraction that makes a big difference.

It’s sort of like their family members. And in fact the number of genes they share in common is so large. And that is almost that they share a common ancestor. It’s like their fourth cousins.

The friends were most alike when it came to their sense of smell but the study suggests that we could be drawn to people with similar traits because it helps us to survive.

If you are the first person that has a mutation and you don’t have any friends, you need surround yourself with other people who also have that trait. This is not just the case that people with the best trait, it’s the people we have similar genes so they are friends.

When it comes to our immunities though, the friends’ genes were more different than that of the strangers. The scientists say this trend could also give us an evolutionary edge though because if your friends can fight different diseases than you, you are less likely to spread infections. It gives a new meaning perhaps to being their bio-friends.

President Putin has ordered a criminal investigation into a crash on a Moscow underground system which left 21 passengers dead. Scores of people were injured, 40 of them seriously when a train derailed. The cause of derailment is not yet known but the authorities have ruled out a terrorist attack. The mayor of Moscow has declared Wednesday a day of mourning. BBC Russian spoke to some of the survivors.

I was inside the carriage. I fell down. The light went off. Everyone was on the floor.

The whole train?

I don’t know. I’ve only seen my carriage. I was in the front carriage.

Was it full?

Not full but half full for sure.