We know so little about fetal development and genetics.
Most twins don't make it - one takes over. I think the literature says only 15% make it.
No one knows how many sets of twins are really Chimeras or how many individuals are really Chimeras. A Chimera is one person who comes from two fertilized eggs whereas identical twins come from one fertilized egg. Right now most genetic testing does not look at many sites on a person.
They are also finding out there are a lot of the mom's and grandmom's and great-grandmom's cells are floating around in kids, too. It is called Microchimerism for now.
