What does chimerism mean?
Chimerism is the condition in which a single organism contains cells from two or more genetically distinct individuals. The name comes from the Chimera of Greek myth, a creature assembled from lion, goat, and serpent. In humans, chimerism can occur naturally — most famously when non-identical twins swap cells in the womb so that each carries some of the other's DNA — or result from medical procedures such as bone marrow transplants and blood transfusions. It also arises routinely during pregnancy, when small numbers of fetal cells cross into the mother's circulation and may persist for decades, a phenomenon known as fetal microchimerism. Chimerism matters practically because it can complicate genetic and forensic tests, sometimes producing results that appear to exclude a person's own biological relationship to themselves or their children. It remains primarily a scientific term, valued for describing one of nature's more counterintuitive biological states.
nounThe presence in a single organism of cells derived from two or more genetically distinct individuals, occurring naturally (for example through exchange of cells between twins in utero) or produced artificially by transplantation or transfusion.
- In biology and medicine, the coexistence within one organism of genetically distinct cell populations derived from more than one individual, arising naturally or through transplantation.
- By extension in genetics, the state or condition of being a chimera — an organism composed of such mixed cell lines.
"Genetic testing revealed that she was her own twin's sister as well — a rare case of natural chimerism."
"Because blood-forming cells from both embryos had mingled in the womb, standard paternity tests initially gave impossible results — classic signs of chimerism."
"Successful organ transplantation depends on a controlled degree of chimerism between donor and recipient tissues."
Usually uncountable; the plural appears mainly in scientific writing referring to multiple distinct instances or types of the condition.
"Both tetragametic and transplant-associated chimerisms were documented in the case series."
Some people carry the DNA of an unborn twin throughout their entire body — mothers can even harbor their children's cells for decades, a phenomenon called fetal microchimerism.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of chimerism
The word derives from 'chimera,' ultimately from Greek 'Khimaira,' the mythical fire-breathing monster composed of parts of a lion, goat, and serpent; the Greek term itself possibly meant 'she-goat.' In biology, 'chimera' was adopted to describe organisms with mixed tissue origins, and 'chimerism' formed from it with the '-ism' suffix to name the resulting condition. Related English forms include 'chimeric' and 'chimerical,' the latter long used figuratively to mean wildly fanciful or illusory.
Related word forms
How chimerism is actually used
A technical term used chiefly in genetics, immunology, transplant medicine, and developmental biology; it is rarely encountered outside scientific contexts. It carries no emotional connotation, though popular accounts often frame it as astonishing because of its implications for identity and parentage testing.
Easily confused with chimerism
Mosaicism arises when genetically different cell lines come from mutations within a single fertilized egg, whereas chimerism involves cell lines originating from separate individuals.