Polar twins

Polar twins or Non-identical monozygotic twins are twins which split from the same zygote but are non-identical. Despite having identical genomes, they develop differently for various reasons.

Polar twins are said to occur when a single egg splits in two and the two halves are fertilized by two separate sperm. The part of the egg that splits off is referred to as the first polar body. This body will usually disintegrate. The result of the two bodies being separately fertilized is a set of twins who have half of their genes in common (from the mother) and the other half different (from the two different sperm). They share some features of identical twins and some features of fraternal twins and are sometimes referred to as half-identical twins.

Polar body twinning is only a theory as there are currently no DNA-based zygosity tests available to determine if twins are polar.

Similar conditions
This condition is similar yet not identical to that of Half Twins but is still often confused.