[Double crossing over: elementary events and the sequence of their occurrence].

Analysis of the crossing over increment in the structurally normal chromosome of Drosophila caused by a rearrangement in nonhomologous chromosome (interchromosomal effect on crossing over, IEC) was carried out based on the author's personal and literature data. The IEC in the left arm of chromosome 2 caused by inversions in chromosomes X and 3, as well as the IEC in X chromosome caused by inversions in chromosomes 2 and 3, were examined. The IEC-induced increment of crossing over results from the increase of the number of double exchanges under the constant or reduced number of single exchanges. Tetrad analysis showed that the given alternation of the crossing over processes could occur only in the case of conversion of the tetrads with single exchanges into the tetrads with double exchanges. In other words, the events leading to the formation of double exchanges occur consecutively. The borders of the IEC-induced double exchanges can be seen all over the chromosome body. However, the IEC-induced increase of chromosome recombination length occurs only in the proximal region (in rare cases, in proximal and distal regions) of the chromosome arm. This means that a double exchange is formed when the first event with predominant location in the middle of the arm is supplemented with the second event predominantly localized at the arm T end, most frequently in the proximal region. The pattern of the IEC-induced double exchange formation can be satisfactorily described in terms of the contact model of the crossing over. According to the model, an elementary crossing-over event is the local contact between the homologues. Neither single exchange nor a double-stranded DNA break can serve as an elementary event in the process of any multiple exchange formation.