Recall the woman who did not seem to be the genetic mother of her children. Upon further testing, cells from different parts of her body were found to be genetically distinct from the cells her doctors had originally tested. The DNA in these other cells matched what one would expect from the mother of the tested children. Scientists determined that this woman is an example of a rare biological phenomenon of chimerism. When her mother was pregnant, there were two fertilized embryos (as you would have in the case of dizygotic twins), however these two embryos implanted very close to one another in the uterus. During the very early stages of development, the embryos merged and the early cells differentiated into endoderm, mesoderm, and ectoderm. Those cells from one embryo differentiated into some portions of the woman’s body, those from the other embryo differentiated into other portions. Because most people never undergo this kind of genetic testing, the frequency of chimerism is not known.
As this case of chimerism became nationally known, it saved another woman who was mired in an uphill legal and custody battle. This second woman had DNA testing done as a routine measure when pursuing child support. The results came back that the children’s father was the biological father, but that she did not seem to be the children’s biological mother. She was accused of fraud and threatened with losing custody of her children. The woman was pregnant at the time and was forced to have a legal witness and immediate genetic testing during the birth of her next child who was also found to not be a match. At this point she was suspected of running some sort of surrogacy scam, and was threatened with further legal action. It wasn’t until news coverage of the first woman’s chimerism that the second woman was more thoroughly tested and found to have chimerism also. Her children were her biological offspring, but she was a chimera between two sibling zygotes that fused in the womb. The judge dismissed all accusations against her. And now this woman can accurately claim to be both biological mother and aunt to her children!