Figure 1:

Potential genomic structures of genetically engineered mice, based on an albino embryonic stem (ES) cell line. The chimeric founder, after injection of albino ES cells into a C57BL/6 recipient blastocyst, is shown in the upper right. Germline transmission of the engineered chromosome (vertical rectangles) is performed by breeding the chimeric founder to a mouse with a different coat color. Coat color is indicated by the mouse cartoon. Percentage of the genome from the ES donor strain and recipient strains are depicted by the circles below the chromosome boxes. Since albinism is recessive, the F1 progeny is crossed back to an albino mouse. Generation of white mice in the second generation indicates successful germline transmission of the ES cell genome (dashed box). Note that the coat color mutation is usually not linked to the engineered locus and therefore they will segregate independently in the progeny. These mice will carry not only the engineered locus (indicated by the asterisk) but also 25% of the black genome. Repeated crossing of the engineered locus back to the black strain results in a congenic animal that is homozygous black for the entire genome except the region surrounding the locus of interest. Repeated brother-sister mating to carry the construct of interest homozygously without first generating congenics can result in novel inbred (recombinant congenic) strains that are composites of the ES cell donor and recipient genomes.