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What Is the Difference Between Monozygotic and Dizygotic Twins?

A pair siblings carried by their mother at the same time are referred to as twins, a common misconception is that twins must look exactly alike. For a pair of twins to look exactly alike they must be monozygotic twins. From a stand point identical twins only occur in one of four conceptions with twins.

Fraternal twins, technically referred to as dizygotic twins, are formed when the woman releases an additional egg while the earlier egg is still in her womb. These two eggs will then be fertilized by different sperm cells, independently. These two eggs shall then grow side-by-side in their mother’s uterus but independent of each other in all other aspects. The word dizygotic comes from the root words “di”, meaning two, and “zygotic”, meaning fertilized egg.

Twins are considered to be identical twins when they are monozygotic. Monozygotic is from the words “mono”, meaning one and “zygote”, meaning fertilized egg. Identical twins look so much a like because they will have exactly the same DNA, since they come from only a single ovum and sperm cell.

Monozygotic twins are formed when a zygote splits apart while it is still in its earliest stage, this usually occurs in the first twelve days of the egg’s fertilization. Monozygotic twins are classified into three different groups based on how old the zygote is before it splits. The differentiation of when splitting occurs is taken note of because this gives an indication of how intertwined the relationship of the monozygotic twins will be.

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