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Twins occur when two fetuses share the same uterus during a pregnancy. Two basic types of twinning can occur (monozygotic (MZ) and dizygotic (DZ)) depending on the number of eggs (zygotes) that are fertilized during pregnancy.
MZ twins, sometimes known as identical twins, are the result of a single egg that is fertilized during conception that then splits into separate embryos. MZ twins, therefore, are 100% genetically the same because they result from a single fertilized egg. This causes them to look very similar and to be the same sex. After they split, these separate embryos develop into two fetuses that share the uterus during pregnancy.
MZ twins may share the same amnion (the inner fetal membrane that contains the amniotic fluid) and placenta. Dizygotic twins, sometimes called fraternal twins, develop when two eggs are released at about the same time and both become fertilized. These eggs then develop into two separate fetuses. Because DZ twins are a result of two different eggs, they are as genetically similar as siblings and they share on average 50% of their genes. Opposite sex twins can occur in fraternal pairs because different sperm fertilize the two eggs, and sex is determined by whether the sperm carries an X or a Y chromosome. Thus, DZ twins do not necessarily look alike and may have differing features as well as similar ones.
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