Imagine if a man discovered one day that two of his three children were not genetically related to him. Recriminations, marital troubles, and perhaps a divorce would be in order. Now, add a twist: What if such a discovery were made by the children’s mother? Suddenly the question becomes not “Who?” but rather “Huh?”

Yet that’s what happened to “Jane”. At the age of 52, when her children were full-grown, she and her children underwent genetic testing for a possible kidney transplant. Completely unexpectedly, two of her three children tested as genetically not hers. A mix-up of babies was ruled out, and Jane and her husband had not undergone in vitro fertilization, so it was absolutely clear that the children she had delivered were actually hers.

Jane, it turns out, is a human Chimera.

The Chimera is primarily known as a creature of Greek legend – a fire-breathing monster with parts of a goat and a lion with a serpent for a tail. In biology, the term has come to refer to any organism that contains more than one set of genes. There are chimera African violets, where the core of the plant is genetically distinct from the outer layers. Animal chimeras, or mosaics, as they can also be called, don’t usually divide so neatly.

The most common form of human chimera is called a blood chimera. This happens when fraternal twins share some portion of the same placenta. Blood and blood-forming tissue is exchanged, and takes up residence in the bone marrow. Each twin is genetically separate except for their blood, which has two distinct sets of genes, and even two distinct blood types. Up to 8% of fraternal twins are blood chimeras.

What happened to Jane is much rarer. Rather than a simple exchange of blood, it was found that she and her fraternal twin merged in utero, leaving only one fetus. The cells in her body are a mosaic of genes from both of the original embryos. The cheek cells from which the genetic testing was done were from one of those embryos, but at least some of the cells in her ovaries came from the other. Interestingly, this genetic oddity gives her a better-than-usual chance of having a successful kidney donation, as her immune system sees two distinct tissue types as non-foreign. She would, however, probably be a poor candidate as a kidney donor, her organs twice as likely to be rejected.

Many human chimeras show no overt signs of their condition, but some exhibit more obvious physical signs. Doctors at the University of Edinburgh in 1998 had a patient referred to them for an undescended left testicle. However, when they examined him they could not find a second testicle. Instead they found something quite unexpected, an ovary and a fallopian tube. Their patient was a chimera formed from the fusion of male and female embryos. While this is a dramatic finding, most chimeras show more subtle signs, such as mismatched eye colors, or parti-colored hair.

This sort of mosaicism is exceedingly rare in the medical annals, with fewer than 100 cases documented world-wide. However, most chimeras are unaware of their condition until some anomaly brings it to light. Given that, it is entirely possible that there are many more chimeras in existence. After all, if Jane’s genetic anomaly had appeared in the father rather than the mother, it is not only possible, but probable, that the first obvious explanation would have been the only one considered, and the affair would have been one for divorce court rather than the New England Journal of Medicine.