Most people are familiar with Identical and Fraternal twins, but in reality, there are 7 different kinds of twins. They are: identical, fraternal, half-identical, mirror image twins, mixed chromosome twins, superfecundation, and superfetation. However, the reason most people are only familiar with identical and fraternal is because the other types are quite rare.
Half-identical twins happen in rare occasions when twins inherit the same genes from the mother, but different genes from the father. Mirror image twins are identical twins that develop mirror-image features (for example, one is right-handed and the other left-handed). Superfecundation is when the mother ovulates more than one egg in one cycle and has more than one partner during her fertile period. Superfetation is the development of a second fetus when one is already present in the uterus.
Identical twins frequently exhibit opposite personalities. Many assume that identical twins (monozygotic twins) have the same, identical DNA, but this is not true. Although they come from the same sperm and egg, science has shown that there are certain points in prenatal development where DNA variance occurs. Also, monozygotic twins do not share identical fingerprints, but have their own unique pattern. However, twins can exhibit very similar brainwave patterns, which may account for anecdotal reports of ESP between twins.
At this time, science doesn't understand why identical twins often develop opposite personalities, but it's most likely due to a combination of DNA variance, environment, and the psychological need of every human being to be unique in some way.
Twin sisters Sophie and Sylvie Morgenstein perfectly exemplify this common trend of opposite personalities. You can follow their lives and help them unravel the disappearance of their brother Davis on societytwins.com.