Can you help me with this using R?

Suppose I'm looking at two groups of people, each consisting of 40 people; you may assume these samples are unpaired. If my first group is drawn from a population where 10% of people have a certain genetic marker, how much higher must the prevalence of that genetic marker be in the population that gave rise to my second group, if I want my study to be 85% powered to detect a difference in the prevalence of this genetic marker between populations? Assume no drop outs.