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.