A strain of bacterial that possesses the F factor integrated into the bacterial genome, hence, when it conjugates with another bacterium, it attempts to transfer a copy of the F factor as well as a portion of or the entire chromosome to the recipient bacterium.
Hfr stands for high frequency of recombination first described by the population geneticist, Luca Cavalli-Sforza. The bacterial cell that acquires F plasmid and incorporates to the bacterial chromosome through crossover, the cell is now designated as Hfr.
During conjugation, the Hfr bacterium would attempt to transfer its entire DNA to the F–bacterium. However, since the mating bridge is structurally fragile the genetic transfer is often incomplete. Hence, the recipient bacterium may only receive a portion of, and rarely the entire, bacterial genome of the donor Hfr bacterium.