 In 1910–1911, Thomas Hunt Morgan (1856–1945) observed several mutants in his stocks of Drosophila melanogaster. When he analyzed these mutants through Mendelian crosses, he found that they were partially linked regarding their inheritance, i.e., the characters were inherited together in more than the 50% of cases that would be expected if genes were randomly distributed, but in less than 100% of the cases, which would have suggested that both characters depend on the same gene. Indeed, Morgan found that his characters could be recombined, despite the fact that they were all sex-linked, i.e., the genes were associated with whatever determined sex in the fruit fly. As every gene was present in two instances (later called “alleles”), the linkage groups could be assumed to exist in pairs and recombination thus suggested exchange between corresponding linkage groups, or “crossing over” of genes. This again suggested that linked genes appeared in linear order, with the frequency of crossing over between two genes being an indicator of their proximity. Other characters were found that also formed such linkage groups but were not sex-linked. Based on these assumptions, Morgan and his students were able to construct maps of four linkage groups in Drosophila that represented the linear order and relative distance as inferred from recombination frequencies (Carlson 1966: Ch. 6–7).