A genetic chimerism or chimera (en-US), also chimaerism or chimaera (en-UK), (/kaɪˈmɪərə, kɪ-/ ky-MEER-ə, kih-) is a single organism composed of cells of different genotypes. Animal chimeras can be produced by the fusion of two (or more) embryos. In plants and some animal chimeras, mosaicism involves distinct types of tissue that originated from the same zygote, but differ due to mutation during ordinary cell division.
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. Innate 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. Cases of human chimeras have been documented.
Most human chimeras will go through life without realizing they are chimeras. The difference in phenotypes may be subtle (e.g., having a hitchhiker's thumb and a straight thumb, eyes of slightly different colors, differential hair growth on opposite sides of the body, etc.) or completely undetectable. Chimeras may also show, under a certain spectrum of UV light, distinctive marks on the back resembling that of arrow points pointing downward from the shoulders down to the lower back; this is one expression of pigment unevenness called Blaschko's lines.
Another case was that of Karen Keegan, who was also suspected (initially) of not being her children's biological mother, after DNA tests on her adult sons for a kidney transplant she needed seemed to show that she was not their mother.
