Genetic Chimerism

In the Iliad Homer described a creature having body parts from different animals, a chimera, from this mythological monster comes the name of the genetic equivalent – chimerism. Genetic chimerism, or tetragametism, in humans and other animals happens when two fertilized eggs or embryos fuse together early in pregnancy. Each zygote carries a copy of its parents DNA and thus a distinct genetic profile. When these merge, each population of cells retains its genetic character and the resulting embryo becomes a mixture of both. Essentially, a human chimera is their own twin.

Chimerism in humans is very rare; Wikipedia states that there are only about 40 reported cases. DNA testing is often used to establish whether a person is biologically related to their parents or children and can uncover cases of chimerism when DNA results show that children are not biologically related to their mothers – because the child inherited a different DNA profile to the one shown by a blood test. This is what happened in the case of Lydia Fairchild: DNA tests of herself and her children led the state to think that she was not actually their mother.

People born with chimerism typically have immune systems that make them tolerant to both genetically distinct populations of cells in their body. This means that a chimera has a much wider array of people to choose from should they need an organ transplant.