Chimera (genetics)
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Typically seen in zoology (but also discovered to a rare extent in human beings), a chimera is an animal that has two or more different populations of genetically distinct cells that originated in different zygotes; if the different cells emerged from the same zygote, it is called a mosaicism. Chimerism in human beings has very few (about 40) reported cases.
Chimeras are formed from four parent cells (two fertilized eggs or early embryos fuse together) or from three parent cells (a fertilized egg is fused with an unfertilized egg or a fertilized egg is fused with an extra sperm). Each population of cells keeps its own character and the resulting animal is a mixture of tissues.
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 a child being a chimera is increased if the child is created via in vitro fertilization. Chimeras can often breed, but the fertility and type of offspring depends on which cell line gave rise to the ovaries or testes; intersexuality and hermaphroditism may result if one set of cells is genetically female and another genetically male.
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