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Twins could be monzygotic i.e. identical twins and dizygotic i.e. non-identical twins.Well, monozygotic twins occur when a single egg is fertilized to form a zygote which later divide into separate embryos.

Now, My question is Why does the embryo collapse splitting the progenitor cell into half? While searching for the answer I found certain theories (possibly without evidence). Following are those theories;

  • An enzyme in the sperm causes them to split.

I have known only a few hyaluronidase and hydrolysing enzymes present in the sperm acrosome which helps penetrating the ovum by lysis of Zona pellucida.

  • A mutation in the mother's RNA which may control embryogenesis in the first nine days.
  • The splitting of embryo is more of a random process then genetically governed one.
  • Genetic mutation which leads to rejection of one group of cells by the other.
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A few possible explanations are named in the Wikipedia article you link:

Regarding spontaneous or natural monozygotic twinning, a recent theory posits that monozygotic twins are formed after a blastocyst essentially collapses, splitting the progenitor cells (those that contain the body's fundamental genetic material) in half, leaving the same genetic material divided in two on opposite sides of the embryo. Eventually, two separate fetuses develop. Spontaneous division of the zygote into two embryos is not considered to be a hereditary trait, but rather a spontaneous or random event.

Monozygotic twins may also be created artificially by embryo splitting. It can be used as an expansion of IVF to increase the number of available embryos for embryo transfer.

Interesting references for this are:

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  • $\begingroup$ +1 For the great answer. According to all the references that you cited either splitting of embryo could be random or it could also be evolutionary as in the case of armadillos.Unlike dizygotic twins monozygotic twins are not a result of hyperovulation then why does IVF increase the chances of having monozygotic twins? $\endgroup$
    – user3800
    Jan 7, 2014 at 2:26

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