Heterozygotes of three-factor crosses of bacteriophage T4

Summary A significant proportion of the recombinants from three-factor crosses between closely linked rII mutants of bacteriophage T4 are heterozygous. Several types of heterozygotes were identified with respect to the type of markers in the heterozygous area. The distribution of the heterozygotes among the types depends on whether the central marker was a deletion or point mutation. In the case of point mutation the majority of the heterozygotes involved the central marker while in the case of deletion most of the heterozygotes involved the double mutation (side markers). The data obtained are explained assuming that recombinants from three-factor crosses arise through insertion of single stranded DNA fragment of the double mutant into the DNA of the central mutant and that extensive correction of deletion heteroduplexes takes place.