What is the cause of negative interference in genetic?

The previous answer is very incorrect. A positive interference number indicates that there is in fact interference occurring that is preventing a crossover event. If the interference is equal to zero, there is no interference. If the interference is NEGATIVE, then one crossover event INCREASES the likelihood of a second crossover event. This is why there are more DCOs observed than expected, i.e. a coefficient of coincidence higher than 1, resulting in a negative interference number.