Twin Zigosity testing is used to determine rather twins are identical, non-identical (Fraternal) or semi-identical (super-fecundation)  Identical Twins develop from one egg which splits early in the development stage, creating twins who share 100% of all genetic material.  Fraternal Twins are the product of two separate eggs fertilized by two different sperm cells creating non-identical twins who share 50% of their genetic material. and sometimes in rare case, two sperm cells fertilize a single egg and create Semi-Identical (super-fecundation) twins.