Who's your Daddy?

There is a sperm donor sibling registry online. I heard about it on the local news the morning.

Turns out that there are many kids who were sired by the same donor. It made me think: A few centuries into the future could a sperm donor have as many desendents asGenghis Khan ? Also, should this registry, or one like it, be expanded to prevent accidental inbreeding? (e.g. Genetic half-siblings meet but don’t know that they’re related.)