An Hfr x F cross involves mating an Hfr (high frequency of recombination) donor strain of bacteria with an F- recipient strain. This results in the transfer of genetic material from the donor Hfr strain to the recipient F- strain. A F+ x F cross involves mating an F+ donor strain with an F- recipient strain. However, in this cross, genetic material is not transferred between the two bacterial strains since neither strain acts as a donor.