ASSOCIATED ALTERATIONS OF RECOMBINATIONAL LENGTH, EXCHANGE NUMBER, AND CHROMOSOME 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 m aterial of the Y chromosome into the 34A region and two inversions, In (1)dl-49 + B-MI and In(3LR)D, demonstrating interchromosomal effects o n crossing over. The length of 2L ranged from 15.0 to 64.9 cM. Interch romosomal effects on crossing over differed in extent and in distribut ion over 2L between females carrying wild-type chromosome 2 and the ch romosome with inserts. Tetrad analysis showed that the 2L recombinatio nal length was increased up to the normal level (females +/+; +/+; +/) or more due to increased proportions of single- or double-exchange t etrads, respectively. A model of pairing and crossing over proposed ea rlier 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 i s determined by the mechanism of contact formation. The extent of inte rference varies both temporally and spatially (within a chromosome), d ecreasing from the periphery to a contact site as the time from contac t formation increases.