How is Zygosity is Determined

Twins often want to know their zygosity (whether they are MZ or DZ twins). While opposite-sex twins are always DZ, the zygosity of same-sex twins may be difficult to determine. The only way to be sure is by doing DNA zygosity testing.

A graph that shows the DNA comparison for a set of identical twins.
Identical Twins

Scientists can determine the zygosity of twins by using a DNA sample from each twin to compare their genetic markers. Markers are well-studied sequences of DNA. A laboratory can analyze specific genetic markers that represent individual regions of DNA that may differ in fraternal twins. These regions are carefully selected, because all humans share mostly similar DNA.

Zygosity testing can be done on cells found in cheeks, blood, amniotic fluid, and other tissues. A twin zygosity test compares the twins’ DNA profiles at a number of markers to see whether they match. If many of these markers are the same in two twins, then it is highly probable (i.e., greater than a 99% chance) that they are identical twins. If the twins are different at one or more markers, they are considered to be fraternal twins. The more markers studied, the more reliable the zygosity results are.

A graph that shows the DNA comparison for a set of fraternal twins.
Fraternal Twins

The laboratory techniques used to determine zygosity are very similar to those used in forensics and in paternity tests. The DNA locations that are analyzed are not related to any specific physical trait such as hair color, height, or personality. Nor are they locations that might indicate whether a person is at risk for a genetic disease.