Chimerism was once believed to be extremely rare, but such conditions are "commoner than we realized," Linda Randolph, M.D., told The New York Times in 2013. Randolph is a pediatrician at Children's Hospital in Los Angeles and author of a chimerism review published in The American Journal of Medical Genetics.
In addition to genetic chimerism, the condition is commonly caused when another genetic code is introduced to the body by organ or tissue transplants, as well as blood transfusions. For example, when someone undergoes a bone marrow transplant, they will carry blood cells that are identical to their donor for the rest of their lives, in addition to their own genetically distinct blood cells. A blood transfusion, however, only produces a temporary chimera condition. The donor blood cells will live about 115 days in the host's body, and will eventually be completely replaced by red blood cells carrying their original genetic code. These types of chimerism are commonly known as microchimerism, and one of the most frequent causes is pregnancy.
Trace fetal stem cells often remain in a mother, long after she has given birth. Some of these stem cells have even been found to reside in women's brains. Likewise, a pregnant woman's cells cross the placenta and become part of their child's liver, heart, thymus gland and bloodstream. Many people may not ever suspect microchimerism is part-and-parcel of their existence, but it's probably more frequent than we realize. In fact, chimerism — both tetragemetic and micro — may someday challenge how medicine, forensics and the legal system contend with DNA.