An Illustration of Twin Zygosity

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Illustrating Twin Zygosity

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Illustrating Twin Zygosity. BSIP/UIG / Getty Images

I've written several articles explaining zygosity, or the classification of twin type. It's an important concept in understanding twinning. Zygosity determines whether twins are identical (monozygotic) or fraternal (dizygotic or multizygotic). It is key to understanding the process of twinning, and how twins form. But sometimes a visualization is much more effective than a written explanation. As they say, a picture is worth a 1,000 words! 

This illustration depicts the formation of both identical and fraternal twins. It is helpful to show a side-by-side comparison of the process by which both kinds of twins form. 

The left side of the picture shows monozygotic twins. They form from a single, fertilized egg that splits. In the picture, you can see the single zygote (egg + sperm). The name is self-explanatory: mono (one), zygote (fertilize egg). It splits into two essentially equal parts, and two babies form. Because they initiate from the same combination of sperm and egg, they often have very similar appearances and may even look exactly alike. Thus, they are commonly known as identical twins. Monozygotic twins are always the same gender (with very rare exceptions). About one third of all twins are monozygotic. No one really knows what causes the egg to split after conception, so the origin of monozygotic twinning remains mysterious. (Note: This illustration depicts monozygotic twins in a shared chorion, with a single placenta. This situation is described as monochorionic twins. However, not all monozygotic twins form this way. Some monozygotic twins develop in separate sacs with two placentas. For more information, see How Many Placentas in a Twin Pregnancy?

On the right side, you see a depiction of dizygotic twinning.  This is commonly known as fraternal twinning.Two eggs are fertilized by two different sperm. Two zygotes produce two embryos resulting in two babies. The process is also known as multizygotic twinning, when it produces higher order multiples, such as triplets or quadruplets. Dizygotic twins are distinguished from monozygotic twins in that they originate from completely separate zygotes. Their genetic background is the same as any two siblings. They develop in separate sacs, with separate placentas. The majority of twins, about two-thirds, are dizygotic. They can be two girls, two boys, or one of each. 

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