In 2002, human chimerism (the condition of being a chimera) was publicized in the popular media with the case of a 52-year-old woman who needed a kidney transplant. To find a potential kidney donor, the woman and her immediate family submitted blood samples for genetic screening. The results were surprising: the tests indicated that the woman did not have biological similarities to two of three sons to whom she had given birth. To solve this conundrum, doctors examined samples of the woman's mucus, hair, and skin and determined that she was a tetragametic chimera; that is, a chimera formed when two of a mother's eggs are fertilized by two of the father's sperm, and the resulting two embryos fuse to form one person.
Microchimerism occurs when a small amount of cells are transferred between the mother and the fetus during pregnancy. Recently, scientists have run studies showing that microchimerism might be very common in humans. In fact, up to 50 percent of mothers may carry their children's cells in their blood decades after giving birth.
A chimera can be defined as an entity (a mixture) which contains two or more genetically different type of cells (cell populations) coming from different zygotes of the same, or different, species.
Conversely, inter-species chimeras, not occurring naturally, have only been artificially generated for scientific purposes. Usually specific technology is needed in order to generate inter-specific chimera in this way. Historically 'probably the first inter-species human chimera was a boy who got a blood infusion from a lamb in 1667 by Jean-Baptiste Denys and who survived this procedure.'
In 1975 evidence of more than one blood type was found in the members of twenty-five fraternal twin pairs. By 1995 only about forty cases had been described, leading researchers to assume that human chimerism was rare. The following year, however, Dutch researchers reported blood group chimerism in 8% (32/415) of fraternal twin pairs and 21% (12/57) of fraternal triplet sets, showing that it occurs more commonly than was realized.
Chimeras, which have mixed cells from two individuals, differ from hybrids, which have combined DNA from two different individuals in all of their body cells.
But creating human-animal chimeras—named after a monster in Greek mythology that had a lion's head, goat's body, and serpent's tail—has raised troubling questions: What new subhuman combination should be produced and for what purpose? At what point would it be considered human? And what rights, if any, should it have?
There are currently no U.S. federal laws that address these issues.
Human Chimera Prohibition Act of 2005 - Amends the Federal criminal code to prohibit and to set penalties for:
(1) creating or attempting to create a human chimera (a being with human and non-human tissue as specified in this Act);
(2) transferring or attempting to transfer a human embryo into a non-human womb, or a non-human embryo into a human womb; or
(3) transporting or receiving a human chimera.
What's more, the incidence of tetragametic chimerism is set to rise, Kruskall says, because of modern fertility techniques that increase the rate of twinning. Drugs used to make a woman ovulate can cause her to release more than one egg at a time, for example, while many IVF clinics still transfer more than one embryo into the womb. And the fact that embryos are in close contact in the lab dish or when transferred to the womb may encourage them to fuse, according to a report by a team at the University of Edinburgh, UK.