An Illustration of Twin Zygosity

Twins form in two different ways

Twinning happens in two different ways. In classifying twins, zygosity is an important term. Zygosity determines whether twins are identical (monozygotic) or fraternal (dizygotic or multizygotic). It is the key to understanding how twins form. But sometimes a visualization is more effective than a written explanation.

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Illustrating Twin Zygosity

Zygosity in Forming Twins
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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 essential elements include the unfertilized egg and sperm. When a sperm fertilizes an egg, it is then a zygote, or fertilized egg. The chromosomes from the nucleus of the egg and sperm form a new nucleus, creating the unique genetic makeup of the new zygote.

The zygote then undergoes cell division to form a ball of cells. It implants in the wall of the uterus, forming a placenta and chorion to be nourished by during pregnancy.

The left side of the illustration shows monozygotic twins, which form a zygote (fertilized egg) from one egg and one sperm. The zygote later splits to form two twins with the same genetic makeup.

The right side of the illustration shows dizygotic twinning, in which each of two eggs is fertilized by a sperm and two zygotes are formed, each maturing into a twin. From the beginning, each has its own genetic makeup, the same as any two siblings from the same parents.

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Monozygotic (Identical) Twinning

Zygosity in Identical Twin Formation
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Monozygotic twins form from a single, fertilized egg that splits. In the picture, you can see the single zygote is created by one egg plus one sperm. It splits into two essentially equal parts, and two babies form.

The name is self-explanatory: mono (one), zygote (fertilized egg).

Because they come from the same combination of sperm and egg, and therefore begin with the same genetic makeup, 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 that 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.

This can be an indication of how early the zygote split to form the twins. Splitting in the first three days can result in each twin having its own sac and placenta. Splitting after that point results in sharing a placenta.

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Dizygotic (Fraternal) Twinning

Zygosity in Fraternal Twin Formation
BSIP/UIG / Getty Images

In this illustration, 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 name comes from the root di (two) and zygote (fertilized egg.) 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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