Scientists have identified a new type of twins that sits somewhere between fraternal twins and identical ones on the scale of similarity. The discovery of the so-called ‘semi-identical twins’ came during an investigation into the genetic make-up of a pair of young twins in the US. The twins are thought to have formed as a result of a previously unknown method, where two sperm cells fuse with a single egg, before becoming two embryos. Each sperm has contributed genes to each child, so while the twins are identical on their mother’s side, they share only half their genes on their father’s side. ‘This probably represents a rare event, but it demonstrates that not all twins conform to the traditionally accepted classifications,’ the researchers said. Study co-author Dr Grant Montgomery, from the Queensland Institute of Medical Research, described it as ‘a rare but interesting finding’. ‘It appears to be an unusual case of twins, and it’s the first to our knowledge,’ he said. ‘It does alert us to the fact that sometimes these rare events occur.’