Twin Zygosity testing is a type of DNA test that is done to determine whether twins are identical twins or fraternal twins.
Identical twins happen when one egg is fertilized by one sperm then it splits into two identical parts. Fraternal twins happen when two eggs are fertilized by different sperm cells but they both attach to the uterus and grow at the same time. The two fraternal twins are as biologically different as two siblings born at different times.
If the twins born are a boy and a girl then automatically that means they are fraternal twins as identical twins are always the same sex. It doesn’t mean that all twins born the same sex are identical twins but they could be.
Most often twins and their parents already know if they are identical or fraternal but they just want to confirm what they already know with scientific evidence. The delivery doctor will also usually make a determination at the time of delivery as to whether the twins are identical or not by whether the two twins developed in the same membrane, however, not all identical twins develop in the same membrane. About a quarter to one third of the time identical twins will develop in separate membranes so sometimes identical twins can be misclassified as fraternal if the doctor sees only one membrane. This is why twin zygosity DNA testing is a good thing to have done if you want confirmation backed up by laboratory testing.
The DNA test done to determine if the twins are identical must show every single marker to be exactly the same. If more than one marker doesn’t match exactly then automatically that means they are fraternal twins. The allowance of one marker is for possible genetic mutations that could happen in the DNA but anything over that allowance is pretty definitive in the results.
Most of the time twins will take a DNA test out of curiosity but it is also a good idea to do this for medical records and health conditions. If one twin experiences a health condition the other might also experience the same health condition. This is not absolute but it is a possibility. There are many identical twins who have very different health issues from each other and different personalities. Life experience will shape different points of view, which has been shown in the study of epigenetics, to be the stronger influence in gene signaling.