Identical (Monozygotic) vs. Fraternal (Dizygotic)

It has always amazed us how many twins have no idea whether they are identical or fraternal. We knew we were identical from the time we could talk. We also had one feature that helped our parents tell us apart, a birth mark.  Here are some interesting facts about twins and being identical vs. fraternal.

The term TWIN refers to two individuals who have shared the same uterus (womb).The biological mechanisms that prompt the single fertilized egg to split in two still remain a mystery.


Identical twins come from a single fertilized egg that splits in two; they share virtually the same genetic code. Any differences between them—one twin having younger looking skin, for example—must be due to environmental factors such as less time spent in the sun. The only difference between identical twins is their fingerprints.  Identical twins develop their own individual personalities to enable themselves to be identified as individual persons. Most people often assume that identical twins are the same person however, that is not the case.

Fraternal twins come from separate eggs and share on average half their DNA. Fraternal twins usually have separate amniotic sacs, placentas, and supporting structures. Some fraternal twins look identical and some just look like siblings. Boy/Girl twins are fraternal. DNA testing will also tell if your twins are identical or fraternal.

Mirror twins also called Mirror Image Twins are a subset of identical twins and are identical twins with opposite features, one is right handed and the other is left handed. One quarter of all identical twins may be mirror twins.

We are Mirror Image Twins and we are both very left and right handed even though we are more dominate by one.

If you aren’t sure if you are identical or fraternal, ask your parents, if they aren’t sure they DNA testing is the only way. However, if you are different sex twins – you are fraternal.

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