How many generations would it take for a single beneficial mutant gene to spread to everyone on earth?

Question

Assuming a 9 billion population and free inter-country, inter-race spread?

Answer 1

It depends on how beneficial the gene is.  The greater the survival advantage, the faster it will spread.

Answer 2

the two beneficial mutations i can think of that would make the greatest impact would be the ability of humans to drink bad water and spoiled food, therefore avoiding diarrhoea, especially in infants and resistance to malaria without the side effects. in third world counties these would spread like wild fire and all though i could not put a number on it, my guess is that it wouldn't be more than 10 generations before this was prevalent in most third world populations. although global warming and the political influence of infant formula manufacturers to avoid prosecution for making contaminated products might make these mutations more beneficial in more developed countries, i think the spread would be slower and come mostly from the infusion of immigrants into developed nations.

Answer 3

Unless anyone who doesn't carry the gene dies, this will never happen. The population is way too large for this to happen.