Final answer:
Zinder and Lederberg concluded that the recovered prototrophs in their transduction experiments were not the result of recombination, based on observations that the frequency of prototroph recovery was the same regardless of the donor strain and that the recovered prototrophs were resistant to phage infection and did not contain any phage DNA.
Step-by-step explanation:
In their transduction experiments, Zinder and Lederberg made observations that led them to conclude that the prototrophs recovered were not the result of recombination. One observation was that the frequency of prototroph recovery was the same regardless of whether the donor strain was a lysogenic or non-lysogenic bacteriophage. Another observation was that the recovered prototrophs were resistant to phage infection and did not contain any phage DNA. These observations suggested that the prototrophs were not the result of recombination, but rather the result of bacteriophage contamination in the experiment.