Linked Genes May Be Mapped by Three-factor Test Crosses

In studying within-chromosome recombination, Morgan proposed that the farther apart two genes were located on a chromosome, the more likely they would be to exhibit crossing-over. Alfred Sturtevant took this argument one step further and proposed that the probability of a cross-over occurring between two genes could be used as a measure of the chromosomal distances separating them. While this seems a simple suggestion, it is one of profound importance. The probability of a cross-over is just the proportion (%) of progeny derived from gametes in which an exchange has occurred, so Sturtevant was suggesting using the percent of observed and new combinations (% cross-over) as a direct measure of intergenic distance. Thus, when Morgan reported 36.9% recombination between w and min, he was actually stating the “genetic distance” between the two markers. Sturtevant went on to propose a convenient unit of such distance, the percent of cross-over itself: one “map unit” of distance was such that one cross-over would occur within that distance in 100 gametes. This unit is now by convention called a “Morgan”: one centimorgan thus equals 0.01% recombination.