Figure 3.

(a) Top line, is the wild-type (WT) allele with a hypothetical exon (box); below that is the floxed allele with the exon flanked by loxP sites (arrow heads), and below that is the floxed allele after Cre-mediated recombination. The location of PCR primers, a, b, and c is indicated along with the sizes of the PCR products. (b) A genetic cross between a mouse bearing Gene X^Cre and Gene Y^Δ/wt and a mouse homozygous for the floxed Gene Y (no Cre). Note that in this breeding scheme, recombination cannot occur in the germline because there is no lox allele in the germline. (c) Hypothetical results from PCR analysis of tail DNA (where recombination is not expected) from Cre-positive offspring. Lanes 1, 2 show expected results with no unexpected recombination in tail DNA. The WT/lox heteroduplex band may appear with too many PCR cycles. Lanes 3, 5 show the results when there is partial recombination (thin arrow Δ) during early development. Lanes 4, 6 show results when there is complete recombination (thick arrow Δ) during early development. The results are the same as when the parent with Gene X^Cre carries Gene Y^lox/wt allele (Figure 2D); however, in that case one does not know whether recombination occurred in germline or during early development. Recombination can also be observed in Cre-negative offspring (not shown). (d) PCR results from a genetic cross of the type shown in B; Note the presence of the Δ allele in lanes 2, 3, and 4. In lane 2, there must have been complete recombination of the lox allele during early development because WT and Δ alleles are not possible from this cross; lanes 3 and 4 reveal partial recombination of the lox allele.