In a collection of cultures all started from a few wildtype

In a collection of cultures, all started from a few wild-type bacteria, which cultures are most likely to have had the earliest mutation to a particular mutant phenotype: those with a few mutant bacteria or those with many mutant bacteria?

Solution

Ans. Imagine a set of cultures inoculated today morning. We analyze the relative number of mutants after 5 days.

If mutation occurs at day 1, the bacteria reproduce and add some progeny to the culture. The resultant bacterial cells will also be mutated and they further reproduce successively.

If a mutation appears at day 5, the bacteria reproduce and add some mutant progeny to the culture. However, the total number of mutants is far less because the resultant progeny was not allowed to reproduce beyond incubation period.

So, earlier the mutation has occurred, the longer period mutants available to reproduce. So, the earliest mutants can add larger number of mutant progeny to the culture, than a recently mutated cell.