Imagine if you discovered one day that two of your three children were genetically not yours. Recriminations, marital troubles, perhaps a divorce, right? Now add a twist. What if you were these 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 she and her husband had not undergone in vitro fertilization, so it was absolute that her children were 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, and as the incidence of fraternal twins in the general populace increases with the popularity of in vitro fertilization, the number of blood chimeras should rise proportionately.

What happened to Jane is a much rarer. Rather than a simple exchange of blood, 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 does not reject as foreign either of two distinct tissue types. She would, however, be a poor candidate as a kidney donor were she in that position, due to the likelihood of two tissue types being present in her kidneys.

Many human chimeras show no overt signs of their condition. Others have more obvious physical findings. 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 eyes, or parti-colored hair.

This sort of mosaicism is exceedingly rare in the medical annals, though not completely unheard of. To date approximately thirty cases have been found 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, in the case of Jane had the genetic anomaly shown up 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 looked at, and the marriage would have ended up in divorce court rather than the New England Journal of Medicine.