Final answer:
The value called 'coefficient of coincidence' is used in genetics to compare observed and expected double crossover frequencies in a three-point cross.
Step-by-step explanation:
When interference occurs in a three-point cross in genetics, the value used to compare the observed frequency of double crossovers to the expected frequency is known as the coefficient of coincidence. This value is a measure of the degree to which one crossover event interferes with additional crossovers in the same region of a chromosome. The coefficient of coincidence is calculated by taking the observed number of double crossovers and dividing it by the expected number of double crossovers based on the recombination frequencies of the individual gene pairs involved.