Chimerism is an extremely rare disorder that mixes the chromosomal population in a single organism. In these cases, chimerism may manifest as the presence of two sets of DNA, or organs that do not match the DNA of the rest of the organism. In some cases, hermaphroditic characteristics, in other words, having both male and female sex organs, can be signs of chimerism. Alternately, small patches of DNA can be present throughout the body.

Chimerism tends to occur very early in the embryonic development. It is often the result of two non-identical twin embryos merging together instead of growing on their own. Although the condition is very rare, with only about 35 people in the US being identified as having chimerism, it tends to get attention from popular media.

Both the television shows, House and CSI have featured episodes dealing with chimerism. In the 2006 episode of House, a young child is found to have microchimerism, small patches of non-matching DNA that are causing the child to become ill as his body rejects the foreign DNA.

In CSI, a suspected criminal has two sets of DNA, one existing in the lower half of his body. Thus his DNA does not match samples found at a crime scene, though other evidence proves him the killer. When the suspect is identified as having chimerism, he is found guilty.

Such cases may make for interesting television, but real cases are seldom identified. There was one legal case involving a woman with chimerism, who was proven not to be the mother of her own children. Later discovery of embryonic cells with different DNA disproved the earlier DNA results.

The number of people with chimerism may not be entirely accurate, since many with chimerism may never show any symptoms that they have other DNA present. Chimerism is most often noted in those considered as hermaphrodites, but not all hermaphrodites have chimerism.

In rare cases, the DNA that produces organs may cause organ rejection or failure if the rest of the DNA in the body attacks the organ as foreign. The odds of this happening are extremely minute.

In some cases, chimerism may be the deliberate result of scientists attempting to mix two species. In one case, scientists successfully produced a goat/sheep mix. They have also been able to produce a chimera quail/chicken. Most often these attempts fail, and if they do work, the animal produced is often sterile.

When scientists attempt to produce chimerism, they are actually merging embryos of two animals. They are not mixing the egg of one species with the sperm of another. That process is more common, more successful and is called hybridization.