The generation of various recombinants by crossing over in different regions

The diagram shows how genes can be mapped by recombination in E. coli. In exconjugants, selection is made for merozygotes bearing the leu⁺ marker, which is donated late. The early markers (arg⁺ and met⁺) may or may not be inserted, depending on the site where recombination between the Hfr fragment and the F⁻ chromosome takes place. The frequencies of events diagrammed in parts a and b are used to obtain the relative sizes of the leu-arg and argmet regions. Note that, in each case, only the DNA inserted into the F⁻ chromosome survives; the other fragment is lost.

ANIMATED ART: Bacterial conjugation and mapping by recombination