Discrete variation in wing morphology is a very common phenomenon in insects and has been used extensively in the past 50 years as a model to study the ecology and evolution of dispersal. Wing morph determination can be purely genetic, purely environmental, or some combination of the two. The precise genetic determinants of genetically based wing morph variation are unknown. Here we explore the genetic basis of wing polymorphism in the pea aphid, which can produce either winged or wingless males. We confirm that three types of pea aphid clones coexist in natural populations, those producing winged males only, those producing wingless males only, and those producing a mixture of both. A Mendelian genetic analysis reveals that male wing poly-