Interference and coincidence
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.
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.