Identical and Fraternal Twins
Twins are either identical or fraternal and are always present within the same pregnancy. The difference between the two types of twins is based upon the difference or similarity in their DNA structures. Medically, identical twins are referred to as monozygotic twins and result when a single sperm cell fertilises a single ovum and the zygote divides at a later stage to form two foetuses. Dizygotic twins results when two sperm cells fertilise two ova in close succession, even on different mating sessions. The womb can be put under considerable pressure with the presence of twins and in fact many twins are born two to three weeks before babies that are not twins. In terms of DNA, monozygotic twins share the same DNA profiles and one cannot distinguish between them by comparing the results of a test. When it comes to dizygotic twins, their DNA is no more similar and no more different to brothers and sisters.
Although many people think that twins can only be identical or fraternal there are actually many other types of twins including parasitic twins and chimeric twins. These latter examples are rather rare and thus, people hear of them less often. Interestingly, some cultures and tribes view twins very negatively and these are believed to be evil. In the Nigerian Igbo culture, any twins born are abandoned at birth.
Twins, whether identical or not, form only around 2% of the world population. DNA testing of twins to find out whether they are monozygotic or dizygotic is useful for a number of reasons. More about twins on Twin Zygosity testing.