Twins have proved indispensable to the progress of science. Till today twin studies of identical and fraternal twins provide valuable insights into the nature of our genes. When a mother gives birth to two offspring in the same pregnancy, we call those offspring twins. Although we think of twins as either identical (monozygotic) or fraternal (dizygotic) there are actually several kinds of twins including parasitic twins conjoined twins, vanishing twins and chimera twins.
Monozygotic or identical twins
From the knowledge which was imparted to us at high school level, we all know that life begins from an egg which is fertilized by a sperm cell to form a zygote. In rare cases, it is found that a single zygote divides into two embryos which results in monozygotic, or identical twins. The two offspring will have same genetic profiles. With the rapid advancement in science and technology, it is possible to have monozygotic twins, artificially, by a technique known as embryo splitting. However, the natural cause of embryo splitting is yet to be known. According to a statistical fact, the birth of identical twins occurs at a rate of about 3 in every 1000 deliveries worldwide.
Dizygotic or fraternal twins
Except fraternal, dizygotic twins are referred by several other sobriquets such as non-identical twins, biovular twins, and sororal twins, which is specially given if both the offspring are female. Sometimes two eggs get fertilized, independently, by two different sperm cells resulting in fraternal twins. The two fertilized eggs or ova form two zygotes, and hence the name dizygotic or biovular. Unlike identical twins, fraternal twins do not share the same chromosome profile. They can either look similar to each other, given their same age, or completely different from each other. They can either be of the same sex or opposite sex. Fraternal twins are just like normal siblings but with no marked age difference.
The occurrence of chimeric twins mostly takes place in animals; however, there were few reports of human chimerism. The concept of chimeric twins is different from that of identical and fraternal twins, as they do not result in the birth of two offspring but one. Chimeric twins in fact, result from the fusion of two separate zygotes which are fertilized by two different sperm cells. If we put it in another way, chimeric twins result from the merging of two fraternal twins. The offspring, born due to chimerism, can be male, female or hermaphroditic. Hermaphroditic is a biological term which refers to those organisms which have reproductive organs associated with both male and female. Chimera twins can have more than one DNA profile and different DNA samples can yield different DNA profiles.
Twin DNA testing
Twin zygosity DNA testing can accurately confirm whether twins are identical or fraternal. As we have seen above, identical or monozygotic twins are formed from the splitting up of the same zygote which was fertilized by a single sperm cell and therefore, identical twins have identical DNA and an identical genotype. However, new studies have shed light over the fact that identical twins may have same DNA, but they may also have different phenotypes (observable physical characteristics). Because phenotypes determine the physical traits of an individual, identical twins will have different fingerprints. This outstanding foundation of biological science had laid the way to a fascinating world of cloning and genetics. Twin DNA testing is useful as being an identical twins has its advantages, to begin their blood groups will be perfectly compatible making them suitable donors to each other. The same applies with organs. Identical twins can donate organs to each other with a much lower chance of the organ being rejected as it is not perceived by the host body as being foreign.