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.