"Real" redheads are a small minority of the population, maybe 4 or 5 percent. To produce a redhead, both parents have to have the redhead gene (melanocortin 1 I think). I think that there's enough of that gene spread throughout the continents to ensure that gingers will maintain their hold in this world.
I'm a natural redhead and my Mom has blonde/brown hair and my Dad's hair was almost black. Not sure what the mailman or milkman had. I wonder if our future wee ones will be redheads. I know Luke has Scottish/Welsh, so there's definitely a chance for that gene to shine through...