In a recent study of chiasma frequencies in the various segments of heterozygotes for complex B-A chromosome rearrangements, in one case (TB-1La5S8041) the frequency was unexpectedly low for the segment representing 20% of the terminal region of the long arm of chromosome 1 (M.P. Maguire, Chromosoma 94:71-85). In fact where a chiasma was present in this region there was almost always a chiasma also present in each of the other arms of the complex configuration, including the opposite arm which is also estimated to be genetically short. This raised the question whether the terminal region of the long arm of chromosome 1 may be essentially devoid of "pairing centers" and depends mainly upon zipping-up of pairing initiated elsewhere for the establishment of crossover pairing. The observation was especially surprising since in another recent study estimates of crossover frequencies in a series of overlapping inversions suggested that the genetic map of the long arm of chromosome 1 is approximately uniformly distributed for the region between about 0.55 and 0.92 of the physical length from the centromere, and crossover pairing is efficiently established in regions heterozygous for these inversions (M.P. Maguire, Genet. Res. 46:273-278). Now the chiasma frequency has been studied in heterozygotes for simple reciprocal translocation T1-4g, with breakpoints located at 1L.95 and 4L.35. Given the fortunate presence of a large chromosome 4 knob, diakinesis configurations were not difficult to classify. For the segment representing the distal 5% of chromosome 1L the total observed chiasma frequency was 8.8% (in 62/704 cells). This is close to expectation from assumption of uniform genetic map distribution throughout the arm. It seems likely that either zipping-up for crossover pairing is very efficient in this system (with pairing facilitation across breakpoints) or "pairing centers" in fact exist in the distal region of the long arm of chromosome 1.
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