We know that twins can be either identical or fraternal. They can of course also be triplets or quadruplets as well as conjoined or chimerical. But for the sake of simplicity and brevity, we shall simply look at the twins we are more likely to encounter or perhaps give birth during our lives identical twins and fraternal twins.
Identical twins are indeed fascinating – seeing two people who are mirror images of each other is quite something. For us unique human beings, these natural carbon copies of each other challenge our concept of uniqueness. We expect people to be different and not to be exact replicas of each other. But twins are actually very different they have different personalities and abilities and are clearly distinguishable on a level of personality. In some cases, even fraternal twins may be strikingly similar or indistinguishable if they are of the same sex of course.
Conception of Twins
Identical twins are conceived when a female ovum is fertilized by a male egg. The egg then splits into two forming two twins. Each new individual carries a copy of identical genetic material given the fact that they developed from the same single fertilized egg. Scientifically, identical twins are referred to as monozygotic twins.
Fraternal twins, that form about 75% of the total incidence rate for fraternal and identical twins, are formed in when two ova are fertilized by two sperm cells. Normally, women only release one egg during their cycle, in some cases, they may however, release two or sometimes more eggs. In some cases, if the mother has had intimate encounters with more than one male, she may conceive fraternal twins with different fathers. Although uncommon, this phenomenon does occur and is known as heteropaternal superfecundation.
We have just mentioned how identical twins or fraternal twins come into existence. Despite identical and fraternal twins being the most common and well know type of twins, there are actually many other types including chimeric twins. A chimeric twin carries the DNA and even the blood group of 2 people. They may have 2 distinct DNA profiles and 2 different blood groups. Chimeric twins come into being when two fertilized eggs fuse together; one egg absorbs the other egg, quite literally. This phenomenon had baffled scientists for a long time; in a well-known case, a mother turned out to be unrelated to her children. In other cases, different tissue samples from the same person yielded different DNA profiles.
Twin DNA testing
Sometimes parents or twins themselves want to determine with 100% accuracy whether they are fraternal or identical. Today, twin zygosity DNA testing is available to help twins or parents of twins get the answers they need. Twin tests are normally done by means of at home kits. All the twins need to do is provide their DNA samples by rubbing the buccal swabs they find inside the kit inside their mouth for around ten or fifteen seconds. Once they have done this, they will need to fill in some forms authorizing the DNA test. Following, they can post their samples for analysis and wait for their results. The results will show the twin's DNA profiles and display the genetic markers tested for each twin. Of course, if the twins are monozygotic (identical) then they will have matching DNA profiles. If they are dizygotic twins, their DNA profiles will be similar but not the same.
But why would anyone want a twins DNA test?
Perhaps because fraternal twins can sometimes look so strikingly similar that you might wrongly assume them to be identical. But there are other reasons.
Organ transplants amongst identical twins have a higher chance of success than when the transplant takes place between normal siblings or close relatives. The chance of organ rejection is greatly reduced in monozygotic twins because their immune systems do not perceive organs donated from each other as alien and thus, do not trigger the autoimmune response. Many times doctors do not need to administer an immunosuppressant or even carry out any type of tissue typing to ensure compatibility between the organ and the receiver. Besides this, identical twins also have an identical blood group which means they are ideal donors for each other.
Twin DNA testing is not a common DNA test, especially given the low incidence rate of twins (be they identical or fraternal). The test can however, help to confirm whether the twin siblings are identical or not which in turn can be useful in those rare instances where a blood transfusion or organ transplant is required.
Karl M McDonald is a free-lance writer specializing in the field of DNA and genetics. The author is a graduate in forensic science who has received his education in both the US and the UK. Karl has currently taken a backseat in the world of academia and dedicated himself to his two kids. He currently works as a free-lance writer from home during his free time. The author normally specializes in writing about genetics and forensic science.