How this calculator works
Observed mutation frequency is the fraction of cells that meet the mutant definition at sampling. It is not a mutation rate per cell division: an early mutation can produce many descendant mutant cells. Mutants and total cells must refer to the same culture and compatible dilution-corrected quantities. Reverse modes produce expected counts, which can be fractional, rather than fabricating integer observations.
Formula
Observed frequency = mutant cells/total viable cells. Mutants per million = frequency × 1000000. Expected mutants = frequency × total cells.
Worked example
Five mutants among 1000 viable cells give a frequency of 0.005, or 0.5%, and 5000 mutants per million. At a 2% frequency, 500 total cells imply 10 mutants.
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Frequently asked questions
Is this the same as mutation rate?
No. A rate describes mutational events per opportunity such as a division or base-generation. Frequency counts mutant descendants at sampling.
What if no mutants were found?
Observed frequency is zero. It does not prove a zero underlying rate; detection limits and sample size matter.
Can I divide plate colonies directly?
Only when dilution, plated volume, viability and selection conditions make the numerator and denominator comparable.