Crossing Over

Recombination events have important uses in experimental and medical genetics. They can be used to order and determine distances between loci (chromosome positions) by genetic mapping techniques. Loci that are on the same chromosome are all physically linked to one another, but they can be separated by crossing over. Examining the frequency with which two loci are separated allows a calculation of their distance: The closer they are, the more likely they are to remain together. Multiple comparisons of crossing over among multiple loci allows these loci to be mapped, or placed in relative position to one another.

Recombination frequency in one region of the genome will be influenced by other, nearby recombination events, and these differences can complicate genetic mapping. The term "interference" describes this phenomenon. In positive interference, the presence of one crossover in a region decreases the probability that another crossover will occur nearby. Negative interference, the opposite of positive interference, implies that the formation of a second crossover in a region is made more likely by the presence of a first crossover.

Most documented interference has been positive, but some reports of negative interference exist in experimental organisms. The investigation of interference is important because accurate modeling of interference will provide better estimates of true genetic map length and intermarker distances, and more accurate mapping of trait loci. Interference is very difficult to measure in humans, because exceedingly large sample sizes, usually on the order of three hundred to one thousand fully informative meiotic events, are required to detect it.