Final answer:
Site-specific recombination must precede transfer of chromosomal genes by specialized transduction, where a bacteriophage transfers specific bacterial DNA near its integration site.
Step-by-step explanation:
Before transfer of chromosomal genes by specialized transduction can occur, the event that must precede it is site-specific recombination. Specialized transduction involves the transfer of a specific piece of bacterial chromosomal DNA that is located near the site of integration by a bacteriophage (phage). This process begins when a prophage is incorrectly excised from the host genome, including some of the host's DNA adjacent to the integration site. The phage then packages this DNA and can transfer it to another bacterial cell during infection. This is different from generalized transduction, which can transfer any piece of chromosomal DNA due to accidental packaging. In contrast to specialized transduction, other methods like conjugation and transformation involve different mechanisms for horizontal gene transfer.