The test process for determining whether twins are monozygotic or dizygotic is called a zygosity diagnosis.

The Center relies on a zygosity diagnosis method based on blood samples. First, blood samples are processed biochemically to extract the DNA, the so-called ”human blueprint”. DNA is a chain-like structure composed of as many as 3 billion links called base pairs. Close inspection will reveal that the base sequences (the way the bases are linked together) in each individual are subtly different.

The zygosity diagnosis focuses on short tandem repeats – short sections in certain base sequences that get repeated several times. The number of repetitions in a base sequence is different for different individuals, but for monozygotic twins the number is the same because both of them have completely similar genes. It can thus be concluded that if twins’ base chain repetitions match they are monozygotic, and if not, they are dizygotic.

In theory, the likelihood that the numbers of base sequence repetitions in two individuals matches by accident is less than 1 in 300 billion, which is why it is considered an extremely reliable method and is also used in the field of forensic medicine and in things such as paternity tests.