The probability of a mutation happens is 10^-8.
In a genome with 3*10^9 positions, what is the probability of it happens at least once, and the probability of it happens in a specific position of the genome ?
The distribution of the number of mutations is binomial with mean $\displaystyle 30$ and sd $\displaystyle \approx \sqrt{30} \approx 5.5$.
So $\displaystyle 1/2$ a mutation is $\displaystyle \approx 5$ sd below the mean, so the probability of $\displaystyle 0$ mutations is essentially $\displaystyle 0$.
RonL