More multidisciplinary research is required for a better understanding of this fascinating subject. If human chimeras are more common than hitherto thought there could be many medical, social, forensic, and legal implications. To date there are no examples in humans of twin chimeras involving germ cells. In marmoset monkey twins the exchange via the placenta is not limited to blood but can involve other tissues, including germ cells. Blood chimeras are formed by blood transfusion between dizygotic twins via the shared placenta and are more common than was once assumed. Only 28 of the 50 individuals with a 46,XX/46,XY karyotype were either true hermaphrodites or had ambiguous genitalia. Even sex-discordant chimeras can have a normal male or female phenotype. Many are discovered accidently, for example, during a routine blood group test. Most chimeras remain undetected, especially if both zygotes are of the same genetic sex. Two zygotes can fuse together during an early embryonic stage to form a fusion chimera. Fetal and maternal cells can cross the placental barrier so that both mother and child may become microchimeras. Natural chimeras can arise in various ways. The first human chimera was reported in 1953. This review concerns natural human chimeras. Recipients of tissue and organ transplants are artificial chimeras. A chimera is an organism whose cells are derived from two or more zygotes. The term chimera has been borrowed from Greek mythology and has a long history of use in biology and genetics.
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