What's the Difference Between Identical and Fraternal Twins?

Hint: It's Not What They Look Like

Twin baby girls laying on bed
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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.

The sources of these stereotypes are easy to find. In stories, on TV, and even in the movies twins are depicted as beingĀ dressed alike and may have similar names.

But there are actually two basic classifications of twins. You may be surprised to learn that identical twins actually represent only about one-third of all twins. Most twins are fraternal, meaning that while they are born at the same time they are only as similar as any other pair of siblings.

Fraternal or Identical? It Depends Upon Zygosity

Twin type 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 multiples. A zygote is a fertilized egg.

Identical twins are the result of a single zygote that splits and produces two children who share the same DNA. Fraternal twins are the result of two unique eggs that are fertilized by two unique sperm. Fraternal twins share neither egg nor sperm, and thus they will not look alike when they are born.

Learn more about zygosity:

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 parts of the fertilized egg implant in the uterus and both produce 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. Fraternal twins may also be called multizygotic.

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. In addition, 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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