Third type of twinning
The medical community has confirmed that in fact there is a third type of twins called half-identical twins, or semi-identical twins.
Genetically and conception wise semi-identical twins fall somewhere between identical and fraternal twins. With half identical twins an egg is fertilized by two sperms and later divided into two embryos. Half-identical twins share at least half of their genes. It is also believed that semi-identical twins share the same genes from their mothers' side (as one egg is only fertilized) and only share half of their genes from their fathers' side.
The chances of conceiving in this manner is very rare though.
