T.H. Morgan and his group found that when genes were grouped on the same chromosome, some were very tightly linked and showed very low recombination (e.g., 1.3%), while others were loosely linked and showed higher recombination (e.g., 37.2%) . Alfred Sturtevant used the frequency of recombination as a measure of the distance between genes, establishing that lower recombination frequencies correspond to genes being physically closer to each other on the chromosome .