[Associated changes in recombination length, number of exchanges, and interference in Drosophila chromosome 2]

A set of eight strains differing in recombinational length of the left arm of chromosome 2(2L) was obtained by means of inserting genetic material of the Y chromosome into the 34A region and two inversions, In(1)dl-49 + BM1 and In(3LR)D, demonstrating interchromosomal effects on crossing over. The length of 2L ranged from 15.0 to 64.9 cM. Interchromosomal effects on crossing over differed in extent and in distribution over 2L between females carrying wild-type chromosome 2 and the chromosome with inserts. Tetrad analysis showed that the 2L recombinational length was increased up to the normal level (females +/+; +/+, +/+) or more due to increased proportions of single- or double-exchange tetrads, respectively. A model of pairing and crossing over proposed earlier was used to interpret crossing-over patterns. The model implies that initial pairing of homologous chromosomes involves the formation of local contacts between them. The number of exchanges in a bivalent is determined by the number of local contacts at the onset of an event termed the determination of crossing-over sites (DCS). Interference is determined by the mechanism of contact formation. The extent of interference varies both temporally and spatially (within a chromosome), decreasing from the periphery to a contact site as the time from contact formation increases.