In a bacterial culture in which all cells are unable to synthesize leucine (leu⁻), a potent mutagen is added, and the cells are allowed to undergo one round of replication. At that point, samples are taken, a series of dilutions are made, and the cells are plated on either minimal medium or minimal medium containing leucine. The first culture condition (minimal medium) allows the growth of only leu⁺ cells, while the second culture condition (minimal medium with leucine added) allows growth of all cells. The results of the experiment are as follows: Culture Condition Dilution Colonies Minimal medium 10⁻¹ 18 Minimal medium + leucine 10⁻⁷ 9 What is the rate of mutation at the locus associated with leucine biosynthesis?