The journal Nature says the twins are identical on their mother’s side, but share only half their genes on their father’s side. They are the result of two sperm cells fertilising a single egg, which then divided to form two embryos – and each sperm contributed genes to each child.
Ron and I had it out a few weeks ago after a tense Nova special about twins if it was possible for 2 sperm to fertilize one egg (yes, we fight about Nova – sometimes I think the things we argue about are luxuries compared to what other couples argue over). Ron took it a step further to wonder if the mom was a skank if two different men could fertilize the same egg. I remain unconvinced since she’d have to be double-teaming pretty fast before the egg secretes the hormone that blocks all other sperm after fertilization. Or using some sort of batter blaster.