In Drosophila, Morgan found that the genes white (w) and yellow (y) showed only 1.3 per cent recombination, while white (w) and miniature (m) showed 37.2 per cent recombination. The best interpretation is that:

Lower recombination frequency means the genes are physically closer on the same chromosome and rarely separated by crossing over. So w and y are tightly linked, while w and m, with higher recombination, are farther apart on the X chromosome.