Normal and inverted phage-by-chromosome crosses. Phages bearing the indicated lac::cat alleles were crossed with wild type (TP829) and lac-inv (TP894) bacteria. Sequence segments are designated as shown in Figure 3. Bacterial sequences are colored blue, phage sequences yellow, and the cat gene white. The percentages of Lac+ bacteria among the chloramphenicol-resistant recombinant progeny are indicated, as the means and standard errors from three measurements. Crosses were done by low multiplicity infection of stationary phase cells. The total yields of chloramphenicol-resistant recombinants ranged from .02 to .04 per infected cell.