After an egg is fertilized, the cells begin dividing rapidly. At this stage, they are undifferentiated and identical. So if you could go to the mass of cells at the very beginning, when there are say 8 cells, could you cut the cell bundle in half, to two sets of 4 cells, and let them keep dividing and doubling into two completely new babies? Could you do this infinitely in a lab? And create thousands of clones from a single fertilized egg?
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Yeah that’s what we call an identical twin. They are also called _monozygotic_ in that they are from one zygote. As it undergoes differentiation, you can get different points at which it splits as well which causes one or two placentas or potentially conjoined twins if they don’t fully split or split very late in embryonic development.
That is how identical twins are born. A zygote splits into two on that early stage and you get two embryos with identical DNA.
Yes. As others have said, this is where identical twins come from.
You can also do the opposite: take two 4-cell embryos and stick them together. They grow into a single person who just happens to have a different genetic code in some parts of their body than in other parts.
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