Why are double-crossover events expected in lower frequency than single crossover events?

Double-crossover events are expected to occur less frequently than single crossover events due to the requirement of two distinct breakages and subsequent rejoining of chromatids within a limited region of DNA. The probability of two crossover events happening in close proximity is inherently lower than that of a single crossover, as the single event can occur independently without the constraints of an adjacent second event. Additionally, the physical mechanisms involved in crossing over, such as the formation of chiasmata, further reduce the likelihood of double crossovers occurring within the same segment.