The methods used to identify the locus of a gene and the distances between genes is called gene mapping. It indicates the % of crossing over occurs between the linked genes. The essence of all genome mapping is to place a collection of molecular markers onto their respective positions on the genome. Recombination between linked genes can be used to map their distance apart on the chromosome. The distance between the two linked genes is measured by centimorgans or map units.
So, the correct option is ‘ratio of crossing over between them’.