Monozygotic semi-identical (polar) twins are a special type of twin. In science it is called an intermediate type between monozygotic (identical) and dizygotic (non-identical) twins. They occur very rarely, and the process of their formation is very complex. Together with the egg, even before fertilization, a polar body, a small cell that usually dies off, is formed.

What is a PolarBody?

A polar body is a small cell that is produced during the process of oogenesis (egg formation) in females. When a diploid germ cell (oogonium) undergoes meiosis to form an egg, it produces three smaller cells called polar bodies and one larger cell, which becomes the egg. The polar bodies do not have the ability to develop into a viable embryo, and they usually disintegrate and are absorbed by the body.

It is believed that in some cases the polar body splits in an unusual way. It increases in size, gets more nutrition and does not die off as usual. Instead, it behaves like a second egg. The polar body and the egg can be fertilized by two different sperm

This produces twins who have about half the same genes (from the mother) and the other half different genes (from the father). They combine both monozygotic and dizygotic traits, so they are also called semi-identical. Unlike monozygotic ideidentical twins, monozygotic semi-identical twins can be of different sexes, which may explain why there are twins of different sexes who look like two peas in a pod.