In zoology terms, a chimera is an animal which has (at least) two different populations of cells, which are genetically distinct and which originated in different zygotes (fertilized eggs). Chimeras are named after the mythological creature Chimera (please kindly refer to my notes below for further details).
Chimerism may occur naturally during pregnancy, when two non-identical twins combine in the womb, at a very early stage of development, to form a single organism. Such an organism is called a tetragametic chimera as it is formed from four gametes—two eggs and two sperm. As the organism develops, the resulting chimera can come to possess organs that have different sets of chromosomes. For example, the chimera may have a liver composed of cells with one set of chromosomes and have a kidney composed of cells with a second set of chromosomes. This has occurred in humans, though it is considered extremely rare, but since it can only be detected through DNA testing, which in itself is rare, it may be more common than currently believed. As of 2003, there were about 30 human cases in the literature, according to New Scientist.
Chimerism is a condition that is clear and distinct from that of Mosaicism, although individuals with the respective conditions, known as chimeras and mosaics respectively, are individuals that have more than one genetically-distinct population of cells. In mosaics, the genetically different cell types all arise from a single zygote, whereas in chimeras, the genetically different cell types originate from more than one zygote. The distinction between these two forms is quite clearly defined, although at times ignored or misused. In mosaics, the genetically different cell types all arise from a single zygote, whereas chimeras originate from more than one zygote.
N.B. In Greek Mythology, Chimera was one of the offspring of Typhon, who was a titan and the final son of Gaia (Mother Earth) and Tartarus (this is apparently both a deity and a place in the underworld which is even lower than Hades itself and this will be further discussed in the following posting), and Echidna who was also known as the mother of all monsters (among her more famous offsprings, other the Chimera include Geryon, the Nemean Lion, Cerberus, Ladon, Sphinx and the Lernean Hydra).
Chimera had the body of a goat, the hindquarters of a snake or dragon and the head of a lion, though other descriptions of her said that she had heads of both the goat and lion, with a snake for a tail. Chimera also breathed fire from one or more of her heads.
Chimera was finally defeated by Bellerophon with the help of Pegasus, the winged horse, at the command of King Iobates of Lycia. There are varying descriptions of her death – some say merely that Bellerophon ran her through on his spear, whereas others say that he fitted his spear point with lead that melted when exposed to Chimera's fiery breath and consequently killed her.