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What's the Difference Between Identical and Fraternal Twins?

Hint: It's Not What They Look Like

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One of the biggest misunderstandings about twins is the distinction between identical and nonidentical (also called fraternal) twins. Many people assume that being identical is an inherent part of being twins, that twins should look and act alike. A common stereotype about twins is that they are exactly alike, clones if you will. Often they are depicted dressed alike and have similar names. But there are actually two basic classifications of twins, and identical twins only represent about one-third of twins.

Zygosity

Twintype has less to do with what twins look like and more to do with how they formed. Zygosity is the term used to characterize the genetic relationship between between multiples. Twins are characterized as identical or fraternal based on zygosity. It is important to understand the concept of zygosity in order to understand the difference between identical and fraternal twins.

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Identical Twins

The correct term for identical twins is monozygotic. Monozygotic twins form when a single fertilized egg splits. (Mono = one, zygote = fertilized egg) Both implant in the uterus and develop as two babies. Identical twins originate from the same combination of cells and have the same genetic origin. They are always the same sex, two girls or two boys. No direct cause of monozygotic twinning has been determined; it is not hereditary. Monozygotic twins represent about one-third of all twins. They may look remarkably similar and it may be difficult to tell them apart.

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Fraternal Twins

Fraternal, or dizygotic twins, are two individuals from the same pregnancy who form from two separate eggs fertilized by two separate sperm. Sometimes the term multizygotic is used also. The genetic similarity between fraternal twins is the same as any two siblings, about 50%. They may look alike, or not, just like any sisters and brothers. They can be boys, girls or one of each. Fraternal twins are the most common form of twins, representing about two-thirds of all twins. Dizygotic twinning can be hereditary and run in families. Most twins and multiples that result from fertility enhancing treatments, such as drugs or procedures like in-vitro fertilization, are dizygotic rather than monozygotic.

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