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The rare case of semi-identical twins (discussion)

This fascinating article talks about the birth of two semi-identical twins about five years ago, this is the second identified case in the world for two twins to be semi-identical. To articulate, semi-identical twins occur when two sperm cells fertilize one egg at the same time. Seems impossible right? And the average biology 30 class will tell you that, however going off of the basic knowledge that is learned in that class, which is, the difference between identical and fraternal twins, genetics seems to always find a way to surprise us. The way that doctors find this extraordinary birth is when they ultrasound the mother, discover that the two amniotic sacs containing the two fetuses are positioning in one placenta (An indication of identical twins) and take another ultrasound later on and find that the twins are different genders, something that absolutely cannot happen if they were identical twins. In 2007, the first case of semi-identical twin was recorded and since then, biologists have reached a conclusion as to how this can occur naturally.

I would be delighted to read about your thoughts on the matter if and when you reply to this thread!
I found this article extremely fascinating as well! From the information I previously learned in Biology 30 regarding the fertilization of an ova and fetal development I was completely unaware that an event like this could occur. After reading the article, I was interested on how it was possible that two separate sperm cells could fertilize a single egg and result in semi-identical twins. With further research I came across another article about the first recorded case of semi-identical twins. This article (,00.html ) proposes two scenarios in which this type of fertilization could have occurred. The first being that the a single egg could have divided before fertilization occurs, this would how ever contradict what we learned in Biology class because the egg is not supposed to divide until fertilization has happened. The other more likely scenario would have been that the egg was fused with two sperm cells and created a triploid cell. During the second-cell stage the gg would then shed the chromosomes from each of the sperm in order to self-correct it's chromosomal count. Overall, a very interesting and rare case.    

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