Before you start flirting with that blonde-haired guy at the bar, think about your spawn, according to scientists who say that they've identified the first gene that is responsible, in part, for gray hair. That gene, dubbed IRF4, is also associated with blonde or lighter-colored hair. That makes sense, because melanin is the pigment that paints hair with the chestnut, golden or raven hues of youth. With age, the melanocytes that produce the color in hair follicles can slow down. And you sta...