Twins Explained Beyond Monozygotic and Dizygotic Twins

In Nigeria, there is a way we identify twins even if they are not identical. Different languages have different names they call their twins but for this quick example, I will use the Yoruba naming of twin which is Taiye/Taiwo for the first child and Kehinde for the second child. Twins are a common thing in my family but the odds of me still having a twin is low even though it is common in my family. To explain the nature of how common twins are in my family, using the yoruba analogy, my mom's maiden name is Kehinde and her father's name is Taiwo. Both in her extended and nuclear family, the total number of twin I have seen is 7. My paternal grandfather's name is Taiwo and three of my aunts have twins but then the statistics of live birth involving multiple fetuses is just about 3%.

The majority of twins are dizygotic or fraternal twins which are twins that aren't identical physically and it happens because the woman released two eggs instead of one which were fertilized together and implanted. Another set of twins which is less common is the monozygotic set of twins or what is referred to as identical twins. This particular twin share most of their DNA together and this is because they come from a single fertilized egg that split into two. Asides these two types of twin that we are familiar with, there are a few other type of twins that you might have not heard or come across before, so let's discuss them quickly.

^pickpik

Have you heard of mirror image twins? This twins are physical mirror images of one another where if one has the right hand as the dominant hand, the other would have the left hand as the dominant one. Also, they can also mirror organ in what is refered to as Situs Inversus, although this is not common. Scientists agree that this occurs as a result of he timing of the fertilization and splitting of the egg because during the first couple of weeks during embryonic development, the process called left-right asymmetry occur where the location of organs are programmed.

Scientists agree that a full mirror image twins occur around two weeks after fertilization when a single embryo splits after the the left-right asymmetry has occurred. There are still a lot around this because while ultrasound works well, it hasn't been able to get a clear picture of what happens during embryonic splitting. So we are still learning about this and hopefully we get a clearer picture over time.

Conjoined twins are another type of twins and there is a high chance you have heard of this in this past where twins have part of their body attached at birth. We are not sure how conjoining occur completely but scientists still go with the fission story explanation where incomplete splitting causes conjoined twins. Another theory agrees that the splitting occurs completely but as the embryo grow, they collide and continue to grow together.

^Wikimedia

About 10% of these conjoined twins are External heteropagus twins where one twin doesn't develop one part of their organ so they have to share one organ. This has been attributed to lopside splitting of one twin causing the other not to develop properly for survival or being attached to its twin. We have not been able to witness firsthand how fission happens so we are still on theories but we have been able to get enough evidence with fission to come out with the theory.

Chimeric twin is another type of twin where one can be ones own twin without knowing it which is based on a condition known as vanishing twin syndrome. In this case, at this early stage, one of the twin disappears because one of the twin dies and the other twin absorbs it in a phenomenon known as Washing Twin Syndrome (don't worry, scientists like to give a name to any phenomenon). When the baby is born, it has the genome of two people which is known as Chimera twins.

When you here some types of twin, beyond the two common ones that you know, just relax and know that there are more types of twins that researcher are discovering and we are working everyday to understand twins.

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^REFERENCE

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^{https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5603179/
https://www.smithsonianmag.com/science-nature/brief-history-twin-studies-180958281/
https://science.howstuffworks.com/life/genetic/chimerism-be-own-twin.htm
https://www.cambridge.org/core/journals/twin-research
https://ijdb.ehu.eus/web/paper.php?doi=10.1387%2Fijdb.082602hk
https://www.theatlantic.com/science/archive/2019/03/these-twins-are-neither-identical-nor-fraternal/584092/
https://link.springer.com/article/10.1007/s10815-013-0109-8
https://www.nlm.nih.gov/hmd/conjoined/embryology.html
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6509934/
https://www.scientificamerican.com/article/3-human-chimeras-that-already-exist/}