As an introduction, the males of the Green Swordtail (Xiphophorus helleri) and the Montezuma Swordtail (Xiphophorus montezumae cortezi) and their hybrids are presented. The courtship patterns of the two species are very different. The F[1] hybrids show behavioural patterns of both parental species. When sexual motivation is low, courtship behaviour is similar to Xiphophorus montezumae cortezi; higher sexual motivation produces courtship behaviour exactly like Xiphophorus helleri. On the example of sword-bending during courtship, the ratio of separation in the F[2] generation is demonstrated using trick photography. The results shows that the behavioural differences are controlled by polygenes.