DOUBLE CROSSOVER IN THE HUMAN XP YP PSEUDOAUTOSOMAL REGION AND ITS BEARING ON INTERFERENCE/

Most models on crossover and crossover interference have assumed that the intensity of interference depends inversely on the physical distan ce separating the respective intervals. The possibility, however, also exists that interference depends on genetic rather than on physical d istance. As the human pseudoautosomal region (PAR) on Xp/Yp is physica lly small, yet genetically a hot spot of recombination in male meiosis , studies on the existence of multiple crossover events may be particu larly useful in addressing the question of interference. Our results d emonstrate for the first time a double crossover during male meiosis i n the human PAR on Xp/Yp. The occurrence of a single obligatory recomb ination event in this region can, therefore, no longer be taken as a d ogma. However, double crossover events seem to remain exceptional and, thus, the model originally suggested by Burgoyne remains globally cor rect. As both recombination events can be localized within a narrow ra nge of physical distance, theories describing interference due to ster ic hindrance may have to be reconsidered. This finding may, therefore, have general implications for our understanding of the mechanism of p ositive and negative crossover interference in mammalian genomes and m ay be interesting especially for linkage mapping where double recombin ations in small intervals tend to be considered as genotyping errors.