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Chimera (in Greek mythology) is a fire-breathing female monster with a lion’s head, a goat’s body, and a serpent’s tail. In biology, a chimera is an organism containing a mixture of genetically different tissues. Can a healthy person be a chimera?
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Very rarely, a person can be dimeric with two different DNA fingerprints. In this person, a blood sample for example can have a DNA fingerprint different than that of a tissue sample. This can take place if one of two fraternal twin embryos dies and is absorbed by the other twin in utero.
In one episode of CSI, the rapist was acquitted based on a negative DNA fingerprint testing despite other evidence. He was discovered to be chimeric only after he had committed another crime, shocking the investigating team.
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This condition is not a science fiction. It happens in real life. Please see this Wikipedia article: Lydia Fairchild – Wikipedia. A woman is denied her already established motherhood because of a negative genetic test. The truth is that the woman is a chimera of two fraternal twins. She is two persons in one.
Many may live with this condition, but not identified. No one would suspect any “genetic abnormality” unless there was some striking observation, e.g., having two different-colored eyes or having male and female organs. It is said that the incidence of such a condition should be increasing due to the increased use of fertility drugs and increased twin conception.
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The aforementioned cases are examples of tetragametic chimerism. An embryo is formed from two different ova and two different sperms, with two cell lines.
Chimerism most commonly occurs when a pregnant woman absorbs a few cells from her fetus, or a fetus absorbs a few cells from its mother. These cells may travel into the mother’s or fetus’s bloodstream and migrate to different organs. They may remain in a mother’s body or a child’s body for a decade or more following childbirth. This condition is called microchimerism. The cause, physiology, and consequent pathology of this phenomenon are not well-settled.
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Artificial chimerism occurs by tissue and organ transplantation. A recipient of bone marrow transplant has a second cell line different from the original (host) one.
As to natural chimerism, twin fusion is rare. The twins are usually tetragametic but may be trigametic. Blood chimeras are formed by blood transfusion between dizygotic twins via the shared placenta and are more common than was once assumed.
Here is a very good review of the subject: Natural human chimeras: A review.
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