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While you’re still just an embryo, you might pick up cells that aren’t yours. If your embryo absorbs the new cells, they can stay with you for the rest of your life. One way for this to happen is if you have a twin. By chance, some of their cells may have ended up in you, or vice versa. This can happen when cells pass from a baby through the placenta. Sometimes they end up in mom, and sometimes they end up in their sibling The tissues which become chimeric in this way vary. It depends on which cells pass through the placenta and where those cells end up. However, blood chimerism is likely one of the more common results. The cells that form the marrow are tough and good at moving around. They are more likely to survive the trip through the placenta from one twin to the other. As many as 8% of non-identical twins may have blood chimerism. For triplets, the chances of being a blood chimera are even higher, increasing to 21%.1 But what if you don’t have a twin? Can you still be a chimera? Actually, yes. Sometimes, chimerism is the result of something called vanishing twin syndrome. Multiple embryos happen in about 5% of pregnancies.2 Usually, this leads to twins... but not always. About a quarter of the time, one of the embryos fails to go to term.2,3 If this happens, the embryo is sometimes reabsorbed into the mother. It might also fuse with its twin. An embryo which fuses this way becomes a chimera. It is not clear how often chimeras are born this way. Based on the rates of multiple embryos and vanishing twin syndrome, as many as 1 in 80 people may be born from a vanishing twin pregnancy. However, not all of these people will be chimeras. If the “vanished twin” reabsorbs into the mother, the baby that is born will not be a chimera. Since the two embryos did not fuse together, all of the baby's cells will have his or her own DNA. Chimerism due to vanishing twin pregnancies may be getting more common. In vitro fertilization (IVF) leads to more pregnancies with multiple embryos. Vanishing twin syndrome is also more common in these pregnancies. As many as 1 in 10 people born by IVF may have had a vanished twin. But again, this does not necessarily mean they will all be chimeras. Two examples in this video seems impossible based on blood type. But chimerism makes it possible. The parent and the child’s blood types are being determined by DNA from two different people. In this way, chimerism can make almost any “impossible” pattern of inheritance possible. It turns out that the rules of blood type can be broken in several other ways. Type O blood sometimes shows up where it is least expected, if one parent has the Bombay blood group. In other cases, a parent with AB blood can unexpectedly have AB or O type children, if they have the cis-AB allele.
