You probably picture fraternal twins, which develop when two eggs are fertilized, or identical twins, which develop when a single fertilized egg splits, when you think of twins. Did you know that there are more uncommon and distinctive sorts of twins? See our list of these unusual and rare twin types, which are pretty uncommon in our twin world. What occurs when an egg splits in two and each piece then encounters a sperm? Theoretically, that is what happens when polar bodies or “half-identical” twins form. These twins share about 75% of their genetic DNA, which makes them very similar but not a perfect DNA match. (More than fraternal twins, but less than identical twins.) Polar body twinning is still only a theory at this time. Although there have been confirmed cases, there are currently no reliable tests to determine whether you are carrying polar body twins. This particular type of twin identifies as being genetically identical on the mother’s side but only having 50% of their father’s genes! When two sperm fertilize a single egg, a triploid is created, which splits into semi-identical twins. One hermaphrodite twin, or person born with a difference between their internal and external genitalia, was raised as a female with both testicular and ovarian structures, while the other was anatomically male, according to a case study published in the Journal of Human Genetics in 2007. Did you know that identical twins develop from a single zygote with either male (XY) or female (XX) sex chromosomes? This explains why identical twins almost always share the same gender. However, there have been a few instances of genetic mutations in which one male twin loses a Y chromosome and turns into a female.