Zygosity Facts

Most people know that there are identical twins and fraternal twins, but do they really know the differences between the two?  Do you know that there could be a third type of twin? Identical or monozygotic twins are formed when a single fertilized egg splits into two eggs. Fraternal or dizygotic twins form when two eggs are fertilized by two separate sperm. The third type half-identical, or polar body twins, occur when a single unfertilized egg splits before fertilization and is then fertilized by separate sperm.

Here are some facts about identical, fraternal and half-identical twins:

Identical Twins

  • Only one-third of all twins are identical.
  • Contrary to popular belief, identical twins do not “run in the family.” No hereditary influence for identical twinning has yet been identified.
  • May have one shared placenta, two separate placentas or two placentas fused into one.
  • They share 100% of their genetic markers.
  • Are always the same sex.
  • Have the same blood type.
  • Not caused by fertility treatments or maternal age.
  • Can result in conjoined twins or mirror image twins.

Mirror Image, Identical Twins

  • Only about 25% of identical twins and 1% of the people in the entire world are mirror image twins.
  • Mirror-image twins occur when a single egg divides later in development than typical identical twins - although the egg is still very small when it splits it already has a right side and left side.
  • Mirror-image twins even as babies may display opposite behaviors/developments.
  • Many times their first teeth come in oppositely and their fingerprints are mirror images as well.
  • In rare cases one twin has internal organs on the usual side and the other twin has them on the opposite side.

Fraternal Twins

  • Can be hereditary on the mother’s side. The tendency to hyper-ovulate, or release more than one egg in a cycle is a genetic trait that can be passed from mother to daughter.
  • May have two separate placentas or two placentas fused into one.
  • Share about 50% of their genetic markers, or the same as singleton siblings.
  • May be same sex or male/female.
  • May or may not have the same blood type.
  • Can be attributed to fertility treatments or advanced maternal age.

Half-Identical Twins

  • It is not known what percentage of twins is half-identical.
  • Might have two separate placentas or two placentas fused into one.
  • Share about 75% of their genetic markers, more than fraternal but less than identical.
  • May be same sex or male/female.
  • May or may not have the same blood type.