In stories from Greek mythology, there existed a creature called a Chimera. The Chimera was a fantastic beast made up of parts from multiple animals. The body and head of a lion, another head of a goat, and a snake’s head for a tail. Science now uses the term Chimera to describe a unique and somewhat baffling developmental process of embryonic development, wherein two separate fertilized eggs fuse together to produce a single organism.
Congenital chimerism is most often seen in non-human mammalian species, but has been found in rare instances in humans as well. The process results in an animal that is a mixture of genetic characteristics. Generally the resulting animal is viable and will live a normal life, including being able to breed and produce offspring of their own.
There are two types of congenital chimerism. The most common is called blood chimerism, and occurs when fraternal twins share some portion of the same placenta. The resulting mixing of blood and blood-forming tissue means the twins will be genetically distinct from each other except for their blood, which will have two distinct sets of genes, and may even have two distinct blood types. Blood chimerism is identified by the finding of two populations of red blood cells.
Tetragametic chimerism is the less common of the two types. This genetic condition occurs when two different ova in a female of the species are fertilized by two spermatazoa. In normal development, these ova would both develop through the expected stages and become fraternal twins. In congenital chimerism, however, these two ova fuse together in the zygote stage creating intermingled cell lines. This results in a body that develops all the proper parts, but different chromosomal sets in different parts. Both kidneys in a single organism may have different chromosomes, for instance, and they both may differ from the chromosomes in the organism’s skin.
The resulting organism is referred to as a tetragametic chimera because it is formed from four gametes (tetra meaning four), two ova and two spermatozoa.
In rodent species, congenital chimerism can result in mottled hair coloring, or two different colored eyes. The results of this process in humans is usually less noticeable and often not detected until genetic testing is done for some other, unrelated reason. The New England Journal of Medicine, for example, relates an instance of a woman who was tested for histocompatibility in preparation for a kidney transplant, and this testing suggested that her biological children weren’t hers. Genetic tests revealed the reason for this discrepancy was congenital chimerism. But, except on the genetic level, the woman in this case presented as a phenotypically normal human being. Other human tetragametic chimera specimens may have eyes of different colors, or differently colored hair patches on opposite sides of the body, or other minor differences in symmetry.
Because the tetragametic chimera is created from the fusing of two developing fraternal twins in the earliest phases of development, the resulting singular organism can be male or female. If the blastocysts were of the same sex prior to fusing together, the presence of chimerism can only be detected through DNA testing. Or if the blastocysts were of opposite sexes, the organism may exhibit sexual organs of both sexes to some degree, or exhibit the condition of true hermaphroditism where both sexual organs are present and functional.
Although this condition is rare in humans, recent research has suggested that congenital chimerism may play a role in the pathogenesis of several autoimmune diseases, including lupus. The research has centered on whether the presence of different chromosomes in a single organism can induce a reaction similar to that seen when a transplant is rejected by its host, such as in bone-marrow transplants or organ transplants. This research is incomplete and at the present inconclusive.