Chimera (pI. chimaera)
An organism that contains cells or tissues with a different genotype. These can be mutated cells of the host organism or cells from a different organism or species.
2. An individual animal or plant made up of cells derived from more than one zygote or otherwise genetically distinct. Animals
Although some chimeras do arise naturally, most are produced experimentally, either by mixing cells of very early embryos or by tissue grafting in late embryos or adults.
Experimental chimeras have been used to study a number of biological questions, including the origin and fate of cell lineages during embryonic development, immunological self-tolerance, tumor susceptibility, and the nature of malignancy. See also: Cell lineage

Mixing embryo cells
Two techniques used to form chimeras by mixing embryo cells are aggregation and injection.

Aggregation chimeras formed from embryos of strains A and B are denoted A B, and chimeras developing from two embryos are often known as tetraparental animals.
Up to 15 mouse embryos have been aggregated to form a single giant blastocyst. However, the offspring from such experiments are always normal, so that size regulation must occur after implantation. On average, 50% of aggregation chimeras will be XX XY. Sex determination seems to depend on the relative number of XX and XY cells in the fetal gonads, and in mice most of these animals develop into phenotypic males.