Identical twins occur when a single fertilized egg splits into two. Identical twins look almost exactly alike and share the exact same genes. Most identical twins happen by chance.
Fraternal twins occur when two, separate eggs are fertilized by two, separate sperm. Fraternal twins do not share the exact same genes — they are no more alike than they are to their siblings from different pregnancies.
Identical twins come from the same fertilized egg and, thus, share identical genetic profiles. But according to new research, though identical twins share very similar genes, they are not identical. A typical DNA test performed on identical twins will return results with 99.99 percent similarity. However, DNA from non-identical (fraternal or dizygotic) twins will generally be about 50 percent to 75 percent similar.