Imagine stacking two jigsaw puzzles with different pictures on top of each other.  You shape them and cut them, so now both the puzzles have matching pieces but different pictures.  You then mix all the pieces together, put the pieces together, but you never make the same picture because your pieces are interwoven with each other.

Chimeras have a similar, yet contested, definition.  The concept of chimerism, even, has fluctuated with the course of time.  The word chimerism has roots in Greek mythology, symbolizing a “monster made of incongruous parts.” (Bonnicksen, 27).  The intimidating and evil creatures lived in nature and rarely came in contact with humans, and these renditions are now open to interpretation in artwork by Thomas Grunfeld and Stephan Balkenhol.  Scientifically, there is no solidified definition of chimerism, but it broadly is taken to represent “an organism with cells from different embryonic origins,” or an organism made of at least two genetically different zygotes.  Chimeras can be within a species, or intraspecies, or between different species, or interspecies.


So what exactly does this definition of chimerism mean? Well at the most basic level it means that an organism will have cells from two different organisms. This could be trace amounts, or large masses of cells. In humans this typically happens in situations of twins. Most are “blood chimera’s” meaning they are non-identical twins who shared a blood supply in the uterus. Even those chimera’s who don’t have a twin will have typically received their foreign cells from a twin that died early in gestation. Twin embryos will share a blood supply in the placenta which allows for the movement of stem cells from one embryo to the other. This is where the foreign cells settle in the an embryo.


The first case of a spontaneous embryonic chimera was “Mrs. McK” and she was discovered in spring of 1953.  She donated blood at a clinic in northern England and when the blood was tested in a blood-typing laboratory they discovered the presence of both type A and type O blood. This result is called mixed-field agglutination. The the time of this discovery very little was known about chromosomes and cytogenetics. The only understanding of the case came from studies done with dizygotic cattle that displayed characteristics of chimerism. These cattle shared placental blood vessels and the heifer calf in a pair of male and female cattle twins would typically be infertile. It was later discovered that Mrs. McK had a twin brother who had passed away.

Now although there are very few cases of chimerism throughout history, scientists believe that chimerism is actually quite common. There are very few cases reported because chimerism does not produce noticeable differences in organisms and thus is only discovered when blood typing is done. Although people may appear to be of sound health with singular sets of DNA/cells when scientist delve in deeper they’ve found cases of chimerism. In a study done observing chimerism in twins and triplets approximately 8% of twins and 21% of triplet sets were found to posses some quantity of foreign cells (Boklage, 350).

There are many facets of chimerism that are highly contested on even today, which is why further research is needed to solidify the genetic processes behind chimerism.  In our future blog posts, we will explore social consequences for humans, ethical consequences in research, and medical implications of chimerism.  While the concept of it is still unclear, chimerism frequency in humans may be shockingly higher than expected because the concept may not be as phenotypically clear as one might expect. In addition, because interspecies chimerism is a possibility, future research on the topic needs to abide by ethical and moral guidelines for human research.  Lastly, medical implications such as transplants and transfusions would be the ultimate goal to optimally understand the concept of chimerism, which has clouded the minds of geneticists for decades.

Here’s a video with a quick rundown of some of the information we’ve just presented by Hank Green: SciShow Video

-Tamamna (Tamanna Sahni and Amna Nawaz)


Bonnicksen, A. L. (n.d.). Chimeras, Hybrids, and Interspecies Research (Politics and Policymaking). Washington, DC: Georgetown University Press. Chimeras (pp. 27-59).

Boklage, C. E. (2010). How New Humans Are Made. New Jersey: World Scientific. Twinning and Spontaneous Chimerism (p. 350).

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