Half twins

Half twins are a very rare development in the area of multiple births.

Cause
The cause for the rarity of this condition is that similar to normal twins, the two individuals share the uterus and therefore obviously have the same mother. However what confuses the matter is that the father of the two offspring must be different, and must both fertilise a single egg at exactly the same time. This naturally leads to complications and so is not attainable during normal partnerships. The two offspring's DNA will differ, although they will have received the same DNA from their mother. This often leads to similar but not identical looks. This should not be confused with any hypothetical situation where two different eggs are simultaneously fertilized, in which they would not have the same DNA from the mother.

Results
There have been very few recorded examples of this condition. All instances have resulted in two male half twins, which has led to the assumption that females cannot be half twins, although this statement has been only hypothesised rather than proven.

Similar conditions
This is somewhat similar to polar twins except that the timeline involving the splitting of the zygote differs substantially and therefore leads to varying results. They too will share half a set of genes.