Most people are aware that twins can be fraternal or identical. Current research indicates that there may be a third variety called Polar Body Twinning.
When twins develop, the type of twinning that occurs is based upon how many eggs where fertilized, and how the egg or eggs divided.
Identical twinning occurs when a single fertilized egg divides in half, and each half develops into an identical child. The twins develop into nearly identical individuals because the egg divides after fertilization, therefore each twin shares the same chromosomes.
The technical term for identical twins is Monozygotic Twins. Monozygotic twins are always the same sex and share the same blood type. Identical twins have similar features, this extends to them having the same eye and hair color and well as common features like the same shaped nose or lips.
While idencticl twins share physical features in common, they are still unique individuals.
Mirror Image Twins - These are identical twins whose features mirror each other. It is as if one was looking in the mirror at himself and seeing his reflection, the features in one twin are the reverse of the other twin.
Conjoined Twins, often called Siamese Twins, are identical twins which are physically joined together in some manner. Conjoined Twins can be further classified based upon how they are joined and what organs they share in common.
Whereas Identical twins are both formed from the same egg, fraternal twins develop from two totally separate eggs. Although extremely rare, it is possible for fraternal twins to have different fathers if each male contributed sperm that fertilized a different egg. For this to occur, viable sperm from both donors must be present at the time of conception.
Dizygotic twins (fraternal) are no more similar than they would be in comparison to their other siblings, because they developed from to different eggs, which were fertilized by different sperm. The only reason that they are 'twins' is that they happened to share the same womb at the same time.
When same sex twins are born, it is possible to tell if they are fraternal or identical by having a blood test performed, by comparing features, or by comparing DNA. Identical twins also have identical DNA.
Polar Body Twins
Polar body twinning is believed to occur when an egg divides before fertilization, and thereafter, each half is then fertilized with different sperm. This gives rise to a state identified as half-identical because half the chromosomes that each of the babies received was identical.
Research into the phenomenon of Polar Body Twinning is ongoing, and has not been definitely proven.