Frequency of recombination between gene pairs on same chromosome as a measure of the distance between genes to map their position on chromosome, was used for the first time by 

Linkage is one of the most important features which defy the independent assortment. Crossing over breaks linkage. Linkage can be extensively used in mapping the genes on the chromosomes, considering that the genes are linearly arranged on the chromosomes.

A. Sturtevant used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes and ‘mapped’ their position on the chromosome. The percentage crossing over is considered as map unit.