All the cells of a chimera are derived from the mutant ES cells which were injected into the blastocyst.

• A. True
• B. False
• C. Not determinable
• D. Only partially

Answer: (B)

Chimeras involve two or more genetically distinct cell populations. When mutant ES cells are injected into a blastocyst, the developing embryo becomes a mosaic, with contributions from both the host blastocyst cells and the injected ES cells. So the animal typically carries tissues that are ES-cell–derived and tissues that are host-derived. Not all cells are from the ES cells, though the ES cells can contribute to many tissues and sometimes the germline. Only in special methods like tetraploid complementation would you expect essentially all cells to be ES-derived, which is not the standard chimera scenario. That’s why the statement is not correct.