What is the difference between identical and fraternal twins?
Identical twins (also called monozygotic twins) occur when one fertilized egg splits and develops into two (or occasionally more) fetuses. The fetuses usually share one placenta. Identical twins have the same genes, so they generally look alike and are the same sex. A woman's chances of having identical twins are not related to age, race or family history.
Fraternal twins (also called dizygotic twins) develop when two separate eggs are fertilized by two different sperm. Each twin usually has its own placenta. Fraternal twins (like other siblings) share about 50 percent of their genes, so they can be different sexes. They generally do not look any more alike than brothers or sisters born from different pregnancies. Fraternal twins are more common than identical twins.
Triplets and other higher-order multiples can result from three or more eggs being fertilized, one egg splitting twice (or more), or a combination of both. A set of higher-order multiples may contain all fraternal siblings or a combination of identical and fraternal siblings.
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