What is Chimaera (genetics)?
[[File:ChimericMouseWithPups.jpg|thumb|A chimeric mouse with its [[offspring]]]] [[File:Materno-fetal microchimerism.svg|right|thumb|160px|During pregnancy, a two-way traffic of [[immune cell]]s may occur through the [[placenta]]. Exchanged cells can multiply and may be immunologically active long after birth.]] A '''chimaera''' or '''chimera''' in biology is a single organism (usually an animal) made of two genetically distinct cells. The two types of cells may come from separate [[Fertilisation|fertilised]] eggs or [[zygote]]s.<ref>King R.C; Stansfield W.D. & Mulligan P.K. 2006. ''A dictionary of genetics''. Oxford University Press, p73. {{ISBN|0-19-530761-5}}</ref> Chimeras that originate from distinct [[Fertilisation|fertilised]] eggs fused together are called dispermic chimeras. When, on the other hand, the genetically distinct cell types originate from the same zygote, the organism is called a [[Mosaic (genetics)|mosaic]]. Dispermic chimaeras are formed from at least two fertilized eggs or early embryos fused together. Each population of cells keeps its own [[phenotype]] and the resulting organism is a mixture of the two phenotypes. Microchimerism is a form of chimerism in which a host harbors a small set of genetically distinct cells. This often happens with [[blood transfusion]] or [[Transplant (surgery)|transplants]], but also during [[pregnancy]] due to the two-way cell trafficking between the mother and the foetus.<ref>Gammill H.S. & Nelson J.L. 2010. Naturally acquired microchimerism. ''Int. J. Dev. Biol''. doi:10.1387/ijdb.082767hg</ref> Recently, [[Epigenetics|epigenetic]] chimeras have been discovered in mouse models.<ref>Chanchao Lorthongpanich ''et al'' 2014. ''Science'' '''341''', 1110 (2013); DOI: 10.1126/science.1240617</ref> == Germline chimerism == Germline chimerism occurs when the germ cells (for example, [[sperm]] and [[ovum|egg]] cells) of an organism are not genetically identical to its own. [[Marmoset]]s almost always give birth to fraternal twins. They may have the reproductiv