[3] In 1947, Joshua Lederberg and Edward Tatum discovered that nutritional mutants of the bacterium E. coli, when incubated in mixed cultures, exchanged genetic markers to generate new recombinants, although the mating efficiency was inefficient.
[5] On the other hand, Lederberg had an alternative explanation for the apparent ordered transfer of part of the chromosome.
However, when the Hfr parent was lysogenic, lysogeny (i.e., the prophage) was not inherited by any of recombinants, which were recovered by growing them as colonies on the appropriate agar medium.
The reason is that transfer of the λ prophage into the F− recipient was accompanied by immediate induction of bacteriophage production within the F− cell.
These observations provided evidence that genetic markers was transferred in one direction during conjugation, from the Hfr to F− cell.