Thread: Writing A Book. Need Help To Understand How A Random Gene Mutation Can Logically Spread.

So basically a random mutation develops in Syria that gives a high resistance/Immunity to Neurotoxins (I.E. Sarin gas, nerve gas). What i would like to know is the specifics on how(if it is actually possible in the real world) for this gene to spread and be guaranteed to pass on to offspring, but also require both parents to have at least one copy of the gene for it to be active.

Optionally, if such a gene is possible: what is the rough growth of this gene in a population from a single carrier after 14 generations, or the formula to calculate it's growth?