Why are the recombinants produced from an hfr x f- cross rarely, if ever, f+?

The F factor straightly handover the donor chromosome in the cross. H takes roughly 90 minutes to transfer the chromosome. The big share of the F factor is last to be transmitted and the conjugation tube is very subtle so the probability for a thorough transfer is low. Consequently, when the bacterial strain attempts to inoculate an Hfr plasmid into an F- strain transversely the conjugation bridge through rolling circle replication, it is infrequently completed and typically breakdowns off before the F factor section is totally transferred.