Genetic Analysis using Hfr and F′ Strains of E. coli
When F+ and F− cells are mixed they rapidly form mating pairs and copies of F are transferred from the F+ to the F− cells, converting all the latter to F+. In these crosses not only is F transferred at very high frequency but also segments of the F+ donor chromosome are transferred at very low frequency (10−6 to 10−7) forming cells that are partial diploids or merozygotes; this transfer can be followed by recombination between the segment transferred from the F+ and the corresponding region of the F− chromosome to produce rare genetic recombinants. Some researchers believe that the occasional chromosome transfer observed in F+ × F− crosses is due to rare Hfr cells arising spontaneously in the F+ population; however, since most of the recombinants are F+ and not F− (as they are in Hfr × F− crosses) it seems unlikely that this is the only explanation.
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