Twin Chimerism and Twin Zygosity Testing
Why do we find twins so curious? Most of us seem to invariably stare at these mirror images of each other. Twins are indeed a fascinating phenomenon. This is all the more so when we look at twins from a scientific or genetic perspective. So you might have thought of twins as being mainly identical or fraternal? But there are actually many more types of twins, including parasitic twins, conjoined twins and chimera twins.
Twin chimerism is a very rare phenomenon in human beings and is more common (although still rare) in animals. Chimera twins actually manifest themselves as a single individual and not as two separate ones. For chimera twins to form, the following must happen during conception:
Two ova must be fertilized by two sperm cells. This process of fertilization is of course normal and is exactly how fraternal twins are brought into the world. But with a chimera twins the fertilized eggs do not remain functionally separate or independent, they in fact merge, fusing into each other to form a single chimera twin. This means that the newly formed individual actually has two genetic profiles or two types of DNA. This is because each independently fertilized egg, prior to fusing together, has its own distinct copy of genetic material. But when fusion occurs, two DNAs come together into one individual.
Due to this incredible natural occurrence, chimera twins will often have two different blood groups and of course, two different genetic profiles. Different cells in their body could thus produce different results when analyzed as if they belonged to different people. A DNA test on the hair of a chimera twin might provide a different DNA profile to the DNA tested from a finger nail – this would lead one to think that there are two different individuals involved rather than just one.
Before we look at identical twins on a genetic level and understand the mechanism at conception, we might as well take a quick peek at fraternal twins. As mentioned, fraternal twins are formed when two eggs are fertilized by two sperms cells (they are in fact referred to as dizygotic twins as two eggs are involved. This is implied by the prefix “di” in dizygotic). In terms of their DNA, fraternal twins are no more different or similar to brothers and sisters.
Moreover, fraternal twins can be born or conceived up to 5 days apart or even have different fathers. How is this possible? The phenomenon is known as superfecundation.
Identical twins are rarer and have a far lower incidence rate. The incidence rate varies in different cultures and it appears to be lowest in Asian countries such as Japan and higher in countries such as the USA. In some African cultures such as the Igbos (a culture found in south-west Nigeria), twins are believed to be evil and, especially during pre-colonial times, one twin would be left to die.
Identical twins or monzygotic twins (the prefix “mono” here implying “one”) are brought about when a single egg is fertilized by a single sperm. Following this first stage, the fertilized egg then splits into two separate eggs. Of course, given that the two new eggs are formed from a single egg, they are made up of the same exact genetic material or almost the same. Recent studies have however shown that identical twin DNA profiles actually differ on some points on the genome. Their profiles are nevertheless, still extremely similar and the implications of these finding are far from colossal.
Twin zygosity testing is a type of DNA test aimed at determining the nature of twin “zygosity”. The word zygosity is the adjective which we get from the noun “zygote”- A zygote is the scientific term which refers to the fusion of a sperm cell with a female egg.
Twin zygosity testing helps to confirm whether twins are fraternal or identical by analyzing DNA samples (usually collected using oral swabs). Oral swabs can simply be rubbed inside the mouth of each twin for a couple of seconds and then sent back for testing. If the twins have identical genetic blueprints then they are of course, identical; if their profiles do not match, then they are clearly fraternal. A twin zygosity test is 99.99% accurate at confirming whether two twin siblings are monozygotic or dizygotic.
Helen Burns is a qualified nurse working in the prenatal care unit of a private hospital. She has currently put her nursing career on hold to look after her two young kids. In her part time, she works as a writer specializing on topics relating to child birth and pregnancy. Helen regularly contributes articles to a number of info sites and blogs. More articles by Helen can be found by visiting: homeDNAdirect USA
You Are Here...
Like This Page?