Interference and coincidence

Linkage and Crossing Over in Diploid Organisms (Higher Eukaryotes)
Coupling and repulsion hypothesis
A testcross in maize
Crossing over and meiosis 
Crossing over and chiasma formation
Mechanism of genetic recombination
Crossing over and linkage maps 
Recombination frequencies from a test-cross
Recombination frequencies from F2 data
Interference and coincidence
Linkage maps
Mapping function and poisson distribution
Linkage groups
Chi-square test 
Cytological basis of crossing over
Creighton and McClintock's experiment in corn
Meselson and Weigle's experiment using lambda (λ) phage
Crossing over at four strand stage

If three genes have an order a-b-c, and the recombination values (a-b) and (b-c) are independent of each other, then one should be able to predict the double crossover value between a and c as a product of individual values (a-b) and (b-c).

For instance, if (a-b) = 10% and (b-c) = 20%, the double crossovers in (a-c) region would be 10% of 20% = 2%. However, in practice the value of double crossovers is always less than the expected value. This is explained by the phenomenon of interference, which means that crossing over in one region interferes with the crossing over in the adjacent regions. The degree of interference may vary in different regions. If double crossovers are absent altogether, we would say that interference is 100%, while if it equals the expected value, we would say that there is no interference. Another term coincidence is used to express the same phenomenon.
If double crossovers equal the expected value, there is 100% coincidence, while if no double crossovers are found, coincidence is zero. The coefficient of coincidence can be calculated as follows:

In the example referred to above, expected double crossovers are 0.1 (10%) x 0.2 (20%) = .02 (2%). If the observed value is 1%, coefficient of coincidences, will be 0.5 or 50% coincidence. The greater the coincidence, lesser will be the interference and vice versa.

Contrary to the common occurrence of positive interference in flowering plants, in bacteriophages and the fungus Aspergillus, negative interference was observed. It means that recombination in a particular region, enhances rather than decreasing recombination in the adjoining regions. This is partly due to a different method of reproduction in these organisms. It is postulated that in these organisms effective pairing sties with high recombination frequencies exist.