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

嵌合体一词是从希腊神话中借来的，在生物学和遗传学上有悠久的历史。嵌合体是一种生物，其细胞来自两个或多个受精卵。组织和器官移植的接受者是人工嵌合体。这项审查涉及自然人类嵌合体。第一个人类嵌合体报告于1953年。天然嵌合体可以多种方式出现。胎儿和母体细胞可以穿过胎盘屏障，因此母子都可能成为微嵌合体。两个受精卵可以在胚胎早期融合在一起，形成融合嵌合体。大多数嵌合体仍未被发现，特别是如果两个受精卵具有相同的遗传性别。例如在例行血型检查中偶然发现了许多。甚至性别不一致的嵌合体也可以具有正常的男性或女性表型。在50个具有46，XX / 46，XY核型的个体中，只有28个是真正的雌雄同体或具有模糊的生殖器。血嵌合体通过共同的胎盘在同卵双生子之间输血形成，比以前设想的更为普遍。在mar猴双胞胎中，通过胎盘进行的交换不仅限于血液，还可以涉及其他组织，包括生殖细胞。迄今为止，在人类中尚无涉及生殖细胞的双嵌合体的例子。如果人类嵌合体比以前认为的更为普遍，那么可能会有许多医学，社会，法医学和法律方面的含义。为了更好地理解这个引人入胜的主题，需要更多的跨学科研究。