Genetic Chimerism

The term chimera refers to an organism that contains two (or more) populations of cells with distinct DNA. The name chimera originates from Greek mythology and refers to a mythical monster composed of body parts from a lion, a goat, and a snake. Chimerism causes issues in legal and medical practice (during paternity or maternity testing) as well as in criminology and forensic genetics.

Chimerism is a rare genetic phenomenon that can only be detected by laboratory DNA testing. The result is the discovery that the organs and tissues of the tested individual carry a different genetic makeup. For example, the tested person’s blood contains one set of DNA, while their skin or reproductive organs have a different set of DNA. Most people with congenital chimerism are unaware of their unique genetic makeup and do not suffer from any health complications. Externally, chimerism in animals and humans manifests as different colored eyes (e.g., one eye is blue and the other is green) or variably pigmented stripes on the skin.

There are three causes of chimerism. It either occurs during pregnancy when fetal cells enter the mother’s body, where they can persist for decades, or conversely, when the mother’s cells pass into the fetus. Another possibility for the development of chimerism occurs in the early stages of pregnancy when two eggs are fertilized by two different sperm cells, and the embryos subsequently fuse into one. Today, transplant-induced (artificial) chimerism is common. A patient receives blood-forming cells from a donor, and for the rest of their life, the donor’s DNA circulates in their blood, while the rest of the tissues in their body have their own DNA.