A technique used in bacterial conjugation experiments to allow recovery of recombinant F− cells, while at the same time selecting against (preventing growth of) Hfr donor cells. For example, suppose the Hfr donor strain is susceptible to an antibiotic (such as streptomycin) and can synthesize histidine; the streptomycin locus must be so far from the origin of chromosome transfer that the mating pairs, which inevitably break apart, have separated before the str locus has been transferred. Suppose further that the recipient F− cell cannot make histidine (his −) but is resistant to the antibiotic (Str r). Only His+ Str r recombinants can survive on a medium lacking histidine and containing streptomycin. The desired gene (His + in this case) is called a selected marker; the gene that prevents growth of the male (str s in this case) is called the counterselective marker.
Subjects: Genetics and Genomics.