Then we select str+ leu+ recombinants by growing them in medium containing streptomycin and lacking leucine. But after only two minutes of conjugation, no strr recombinants that are leu+ are formed. This tells us that in two minutes of the transfer no leu genes have been transferred from the streptomycin sensitive Hfr cell to the streptomycin resistant P- cell.

On the other hand, if we allow conjugation for four minutes, a significant number of this recombinant will arise. Therefore, a transfer of the leu genes takes about four minutes after conjugation begins. Taking the procedure a step further, if conjugation proceeds for ten minutes, some leu+ lac+ strr recombinants appear.

Thus, gal would be transferred before lac and leu would be transferred last of all. Even though the order varies, all the genes are in order. For example, leu is transferred first and azi gene is transferred last by Hfr 1, whereas these two genes are transferred one after the other (azi first then leu) in Hfr2, Hfr3, Hfr4. All we must do to make these two orders identical is to form a circle.

Then the only difference will be location of origin of replication, which determines which gene will be transferred first. (Now look at the order of Hfr5, 6 and 7, you will see that the order of transfer of these two genes is reversed leu, then azi. Again, the order of gene transfer by these two strains can be reconciled by forming a circle of each and twining one of them over as you would flip a coin.)