In biology, biotechnology and genetic engineering, a chimera is a hybrid or composite organism created from two or more species that could not have naturally mated and reproduced.
A chimeric organism may be made by introducing cells from the early-stage embryo of one species into another embryo, thus creating a composite animal (for example, a geep) that is not a true genetic hybrid.
A truly genetically-engineered chimera is created by introducing recombinant DNA from one species into the genome of the cells of a blastula (early-stage embryo) of another species. Retroviruses are often used to transform and transfer the DNA into the embryonic cells. Such a chimera's fundamental genetic structure is different from that of any of its parent species. If the genetic alterations are significant, the organism may be sterile, but if it is fertile, it will be able to pass its novel genes on to its offspring.
On the molecular level, a chimera is also a recombinant DNA molecule created by joining DNA fragments from two or more different organisms. Such DNA is referred to as chimeric DNA. A chimeraplast is a synthetic molecule made up of both RNA and DNA which is used in gene repair.
Chimeric mice and rats that contain select genes from other animal species (including humans) are widely used throughout biological and medical research and are particularly popular in the study of genetics, cancer, aging and developmental biology.
Some of the information in this writeup was taken from entries I wrote for the science dictionary at http://biotech.icmb.utexas.edu/.