How did Seymour Benzer use deletion mapping to construct fine-scale maps of the rII region of the T4 ...

He used deletion mutants, which, unlike point mutants, have lost a segment of DNA and cannot revert to a wild-type state. He crossed unknown point mutants with standard (known) deletion mutants for different segments of the rII region to determine which crosses could result in wild-type recombinant phage. Those that could not must have used point mutants with a mutation in the region of the deletion. This could be repeated in crosses between the point mutant and smaller secondary reference deletion mutants until the point mutation was localized to within a very small segment of the rII region.