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
c.o.c. = actual double recombinant frequency / expected double recombinant frequency
Interference is then defined as follows
interference = 1 - c.o.c.
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.
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