How to make a chimera

Boston.com / News / Boston Globe / Health / Science :: CAROLYN Y. JOHNSON: "A chimera is an animal with its own cells and that of another animal growing side by side in its body. Scientists used to call such animals ''tetraparental," meaning they had four parents -- their own, and the parents of the other cells living in their bodies.

The ''geep" is an example of a true chimera. Scientists at the University of California at Davis fused together sheep and goat embryos. In the offspring, every organ, including the sex organs, were made up of both goat and sheep cells, which meant that one ''geep" could produce both goat and sheep sperm.

The proposed ''humanzee" would fuse chimpanzee and human embryos, possibly creating an animal stronger and hairier than a human, but smarter than a chimp.

Many other kinds of chimeras are made by adding human cells to animals that have been genetically altered to have no immune system. Without the immune system, which would attack foreign cells, scientists can grow human brain cells or breast cells in another animal, often a mouse.

Chimeras surround us, and not just in the lab. Cardiac surgery patients who receive pig valves to repair their hearts are chimeras. Transplant recipients, who have another person's bone marrow, kidney, or heart thumping in their chest, are a kind of chimera, with the cells of another being living alongside their own."