Therefore, it relies on deviation from Mendelian laws i.e. independent of assortment. If two loci segregate together most of the time, it is very likely that they are located very close to each other on the same chromosome. Please note that theta has been shown as q below. The classical approach of estimating the distances between pairs of genes is by calculating the recombination frequencies between them. Recombination frequency is the proportion of recombinant types between two gene pairs compared to the sum of all combinations.
Recombination is dependent on the chiasma formation during meiosis.
Therefore, a chromosome with m chiasmata, would have a length of 50 x m cM. Following the calculation of recombination frequencies between each gene pair, the positions of these genes on a chromosome are indicated, i.e. the largest recombination frequency will be between the two outside genes and the remaining gene would be somewhere in between the other two. 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.