Identical twins or as they called monozygotic happen when a single egg, fertilized by a single sperm and divide into two identical halves. And then two separate babies with identical DNA are formed. Identical twins are always the same blood type and sex. They have two separate placentas, but moreover they almost always share the same placenta. Twins are always identical when they share the same amnion and chorion.
Identical twins can be unusual similar in their appearance. But it is not always true due to environmental factors. Twin zygosity test can help to determine if your twins are identical fraternal with staggering accuracy. Such DNA testing is easy done by taking swabs from the mouths of each twin and send to a lab for analysis.
Another curiosity is identical twin fingerprints. They will never be exactly the same. Fingerprints of identical twins will have similar patterns for sure, but there will nevertheless be differences. The scientific community does not have final answer why one egg splits, but they do consider identical twinning to be non-hereditary. Identical twins seem to happen randomly throughout the population without rhyme or reason.
From a scientific point of view, the children of identical twins are the genetic equivalent of half-siblings rather than cousins. So, if identical twins marry another set of identical twins, then the resulting children would be the genetic equivalent of full siblings!
Fraternal twins or as they called dizygotic happen when two separate eggs are fertilized by two separate sperm. Fraternal twins can be the same blood type or/ and sex. They always have separate placentas. But also their placentas may fuse together during the course of pregnancy and appear as one at birth. They will never share the same amnion and chorion.
Fraternal twins can share many similarities in appearance just like any sister or brother of the same family. Fraternal twins are actually the most common type of twinning. Fraternal twins come about due to a number of factors. Because the ovary’s malfunction of releasing two or more eggs at one time may be inherited. It is very common to have a several sets of fraternal twins throughout many generations in one family.