Coefficient of coincidence
In genetics, the coefficient of coincidence (c.o.c.) is a measure of interference in the formation of chromosomal crossovers during meiosis. It is generally the case that, if there is a crossover at one spot on a chromosome, this decreases the likelihood of a crossover in a nearby spot. This is called interference. The coefficient of coincidence is typically calculated from recombination rates between three genes. If there are three genes in the order A B C, then we can determine how closely linked they are by frequency of recombination. Knowing the recombination rate between A and B  and the recombination rate between B and C, we would naively expect the double recombination rate to be the product of these two rates. The coefficient of coincidence is calculated by dividing the "actual" frequency of double recombinants by this expected frequency: Interference is then defined as follows: This figure tells us how strongly a crossover in one of the DNA regions (AB or BC) interferes with the formation of a crossover in the other region. Worked example. Drosophila females of genotype "aa bb cc" were crossed with males of genotype "aa bb cc". This led to 1000 progeny of  the following phenotypes: From these numbers it is clear that the b/b locus lies between the a/a locus and the c/c locus. There are 23 + 152 + 148 + 27 = 350 progeny showing recombination between genes A and B. And there are 81 + 23 + 27 + 89 = 220 progeny showing recombination between genes B and C. Thus the expected rate of double recombination is (350 / 1000) * (220 / 1000) = 0.077, or 77 per 1000. However, there are actually only 23 + 27 = 50 double recombinants. The coefficient of coincidence is therefore  50 / 77 = 0.65. Interference is 1 − 0.65 = 0.35.