Final answer:
Higher frequency gene transfer from Hfr to F− cells happens because the F plasmid integrated into an Hfr cell's chromosome initiates transfer of chromosomal DNA starting near the integration site, causing genes closer to this site to be transferred more frequently.
Step-by-step explanation:
The transfer of chromosomal genes occurs at a higher frequency when cells of an Hfr strain are mixed with F− cells due to the specific behavior of the Hfr cells during bacterial conjugation. An Hfr cell has an F plasmid integrated into its chromosome, allowing it to initiate the transfer of chromosomal DNA to a recipient F− cell. The integration site of the F plasmid becomes the starting point for this DNA transfer. As the chromosomal genes closest to this integration site are the first to be displaced during the process known as rolling circle replication, these genes are transferred and recombined into the recipient's chromosome more frequently than genes located further away. Additionally, because the contact between the Hfr and F− cells is typically transient and the entire chromosome is large, conjugation often ends before the full genome can be transferred, further increasing the likelihood that only genes near the integration site are transmitted to the F− cell.