Breeding Chimeric Mice with Wild Type Mice To Transmit the Mutation
A chimera has the mutation of interest in only some cells of its body, which is not sufficient for research use. In order to make a new GEM, typically many chimeric pups are generated, and then a few of the best chimeras are bred to wild type mice of the same background strain as the ES cell in order to get mice that transmit the mutation through their germline (to their pups). If germline transmission occurs, the pups produced will have the mutation in all cells of their body, not just some cells.
Judging the Quality of the Chimeric Mouse
In order to pick which are the best chimeras to breed, there must be some way to judge quality of the chimera. Good chimeras have a high contribution of cells from the ES cell line. If the host blastocyst and the ES cell line both come from the same background (for example, both are B6), the chimeric pups will be completely black. If the host blastocyst and the ES cells come from strains which have different coat colors, the chimeras will be a mixture of the two coat colors, and good quality chimeras can be judged visually just by assessing what percentage of the ES cell coat color is represented on the mouse.