However, recombination frequencies, over large distances are not additive. The ratio of the observed number of double recombinants to the expected number of recombinants is known as coefficient of coincidence. Because the crossing over in one region inhibits or interferes with crossing over in a neighbouring region, this phenomenon is termed interference. To construct a genetic map, the recombination frequencies are converted into centiMorgans and to do this different mapping functions are used. Haldane's mapping function considers the absence of interference whereas Kosambi's considers positive interference.
Positive interference expects few double recombinants in a given region of chromosome.
Therefore where double recombinations occur, Kosambi's mapping function will give a larger distance between markers than Haldane's. For producing large linkage maps, the linkages between every pair of marker are calculated in turn. Then the order of these marker loci are estimated from the recombination frequencies and these are converted to map distances. However, in a study of a large number of markers, these calculations would be too time consuming and laborious. There are number of computer packages available that can be used to calculate recombination frequencies and to convert them into map distances automatically.