In 1904, Wyndham Dunstan, Director of the Imperial Institute, announced that the British Empire’s equatorial ‘cotton belt’ was waiting to be unlocked by the application of science to agriculture. One application identified by Dunstan was plant breeding, particularly hybridisation and Mendelian genetics. Scholars have demonstrated that one of the early ambitions of British Mendelians was to apply genetics to tropical agriculture in the Empire. Yet applying early-twentieth century genetics to agricultural practice was not straightforward. This paper will evaluate the relationship between Mendelism and the British cotton belt through three case studies. In Egypt, the Khedivial Agricultural Society sought the expertise of American cotton breeders and sponsored the research of Cambridge geneticist William Lawrence Balls. On the Gold Coast and Nigerian Protectorates, imported varieties and agricultural education took precedence over breeding. In Uganda, British Governor Henry Hesketh Bell took the drastic step of destroying cotton. Only by eliminating variety in favour of a single species, argued Bell, could Uganda host a cotton industry. These cases suggest that Mendelism was invoked more as a future ambition, or post hoc explanation for success. Ideas of degeneration, purity, and even eugenic thinking were more influential in shaping the British cotton belt.