Final answer:
An Hfr cell is a bacterial cell in which the F plasmid has integrated into the bacterial chromosome, allowing for the transfer of chromosomal DNA during conjugation. The genes on the Hfr cell's genome can be mapped based on when they are transferred through conjugation.
Step-by-step explanation:
An Hfr cell is a bacterial cell in which the F plasmid has integrated into the bacterial chromosome. This integration allows for the transfer of chromosomal DNA to a recipient cell during conjugation. The integration site of the F plasmid determines which genes are most likely to be transferred and recombined into the recipient cell's chromosome.
The transfer of the entire bacterial chromosome takes a long time during conjugation, so it is more common for only a portion of the chromosome to be transferred. The relative location of bacterial genes on the Hfr cell's genome can be mapped based on when they are transferred through conjugation. This mapping technique was used before bacterial genome sequencing became widespread.