Morgan and his students became more successful at finding mutant flies; they counted the mutant characteristics of thousands of fruit flies and studied their inheritance. As they accumulated multiple mutants, they combined them to study more complex inheritance patterns. The observation of a miniature-wing mutant, which was also on the sex chromosome but sometimes sorted independently to the white-eye mutation, led Morgan to the idea of genetic linkage and to hypothesize the phenomenon of crossing over. He relied on the discovery of Frans Alfons Janssens, a Belgian professor at the University of Leuven, who described the phenomenon in 1909 and had called it "chiasmatypie". Morgan proposed that the amount of crossing over between linked genes differs and that crossover frequency might indicate the distance separating genes on the chromosome. The later English geneticist J. B. S. Haldane suggested that the unit of measurement for linkage be called the morgan. Morgan's student Alfred Sturtevant developed the first genetic map in 1913.