Different Types of Twins: Identical, Fraternal, or In-Between?

Parents of same sex twins always wonder if they have identical or fraternal twins. Without a DNA test there is no absolute way to tell. Even if your children each had their own sac and placenta they can still be identical, although it is more common for identical twins to share a sac.

Now there is a third option, semi-identical twins. Geneticists have called this condition a kind of mix between identical and fraternal twins. Semi-identical twins occur when one egg is fertilized by two sperm. Both twins have identical DNA from the mother, since they shared the egg, but they have different DNA from the father, since they came from two separate sperm.

It is thought to be very rare for an egg to be fertilized by two sperm and then separate to form two embryos. For identical twins, one egg is fertilized and then splits, forming two identical embryos. For fraternal twins, two separate eggs are fertilized by one sperm each, forming two embryos.

The concept of semi-identical twins was only a hypothesis until recently. A pair of semi-identical twins was found by doctors when they were treating a young pair of twins. One was male and one had both male and female genitalia.

The concept of a new type of twins will probably take some time to catch on. If companies make DNA testing available to detect this new category, parents can know for sure what type of twins they have.

As science continues to unravel the genetic mysteries of twins we can continue to unravel the domestic mysteries of twins (how exactly do you keep twin toddlers entertained and safe long enough to take a bathroom break...).