Identical (monozygotic or MZ) twins form when a single, fertilized egg splits in two, so identical twins share 100% of their genes and are always the same sex. There is no known genetic factor that influences the likelihood of having identical twins, and the frequency of identical twin births is approximately the same worldwide. Identical twin pairs occur in about 1 in every 250 births.
Fraternal (dizygotic or DZ) twins form when two eggs are fertilized by two separate sperm. Fraternal twins share, on average, 50% of their genes and may be the same sex or opposite sex. Having fraternal twins is a trait that runs in families, typically on the mother’s side. Fraternal twin births are also more common with higher maternal age and the use of fertility treatments. Given the various factors that can contribute to fraternal twinning, the frequency of fraternal twin births varies throughout the world. In the U.S., fraternal twin pairs occur in about 3 out of every 100 births.
For more information on twin zygosity and factors that may influence twinning, please see this informative article on the Stanford at the Tech Museum website.
We often receive questions about zygosity testing for twins. While we do not offer this service, there are many companies that provide in-home testing, with a range of prices and DNA collection methods. Please note, we do not necessarily endorse any of these companies; these links are provided to help in your search for the best solution for you and your family.
The Genetic Testing Laboratories, Inc.
Proactive Genetics, Inc.