Chimerism is the fascinating state of a fully-formed organism having two distinct sets of DNA.  Basically, during development, multiple (usually just two) zygotes fused and developed into a single embryo. The allocation of the DNA can happen at a high level, such as the top half of the body is DNA1, and the bottom half is DNA2. In addition to, or instead of that, the DNA allocation within individual organs can be a fairly even mix of the two sets of DNA or it can be split essentially down the middle.

The incidence of chimerism among humans is estimated to be very low, but here’s the spooky part: Consider how rarely people would ever get a DNA test to see if they had more than one set of DNA. Maybe you are a chimera and don’t know it.

We have technologies to test for known genetic diseases, but if you’re a chimera, you might provide a mouth swab to test for a disease, and get a negative result, except it’s only negative for that set of your DNA. Maybe the other set of your DNA has the bad genes but wasn’t tested. False positives could occur, too, of course. I also wonder if chimeras could be vulnerable to compound diseases that involve the two sets of DNA interacting in an incompatible way.

As we enter an era when DNA testing for medical and legal purposes seems only to be on the rise, it’ll be interesting to see if chimerism is discovered to occur at much higher frequency than previously estimated.  If so, chimerism may need to be the first DNA test done before doing any further DNA tests on an individual.