Using recombination frequency to create genetic maps. Sturtevant created genetic maps showing the positions of genes along the four Drosophila chromosomes, based on the frequency of crossing over between many pairs of linked genes. Linked genes fall into four groups, corresponding to the four different chromosomes in Drosophila, represented here by the vertical lines. Numbers on the left side of each chromosome are genetic map units, in centimorgans. Along the right side are the names of mutant alleles used in the crosses.