Would consist of binding trades of genetic material between two potential allies. Their future offspring would then be a blend of both parents.
Twins already have a huge advantage, in it being rational for them to be really altruistic. That's something that non-twins can't share. If there were a way to trade genetic material as an adult, as a promise to include some part of them in future generations, you'd have it.
The belonging ritual of becoming blood brothers is a primitive form of this.
What if you encrypted DNA and then had some protein which was the decryption key, in some kind of public key genetic cryptography? It could be a way to secure these genetic investments.
What if the mafia kidnapped someone, and implanted cloned testicles in him, so that his children wouldn't actually be his own? That'd be the ultimate form of retribution.
There's an old science fiction story about a mad scientist who creates a virus which infects women and makes them pregnant with his genome. It gets out of control, and over the next few years, hundreds of millions of his clones are born in every country in the world.
Later I found out this is actually called the "green beard" phenomenon.