Genetic chimera

A genetic chimerism or chimera (also spelled chimaera) is a single organism composed of cells from different zygotes. This can result in male and female organs, two blood types, or subtle variations in form. Animal chimeras are produced by the merger of multiple fertilized eggs. In plant chimeras, however, the distinct types of tissue may originate from the same zygote, and the difference is often due to mutation during ordinary cell division. Normally, genetic chimerism is not visible on casual inspection; however, it has been detected in the course of proving parentage.

Another way that chimerism can occur in animals is by organ transplantation, giving one individual tissues that developed from two genomes. For example, a bone marrow transplant can change someone's blood type.

An animal chimera is a single organism that is composed of two or more different populations of genetically distinct cells that originated from different zygotes involved in sexual reproduction. If the different cells have emerged from the same zygote, the organism is called a mosaic. Chimeras are formed from at least four parent cells (two fertilised eggs or early embryos fused together). Each population of cells keeps its own character and the resulting organism is a mixture of tissues. There are some reports of human chimærism.

This condition is either inherited or it is acquired through the infusion of allogeneic hematopoietic cells during transplantation or transfusion. In nonidentical twins, chimerism occurs by means of blood-vessel anastomoses. The likelihood of offspring being a chimera is increased if it is created via in vitro fertilisation. Chimeras can often breed, but the fertility and type of offspring depends on which cell line gave rise to the ovaries or testes; varying degrees of intersex differences may result if one set of cells is genetically female and another genetically male.

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