Abstract
The frequency of crossing over per unit of physical distance varies systematically along the chromosomes of Drosophila melanogaster. The regional distribution of crossovers in a series of X chromosomes of the same genetic constitution, but having different sequences, was compared in the presence and absence of normal genetically mediated regional constraints on exchange. Recombination was examined in Drosophila melanogaster females homozygous for either normal sequence X chromosomes or any of a series of X chromosome inversions. Autosomally, these females were either (1) wild type, (2) homozygous for one of several recombination-defective meiotic mutations that attenuate the normal regional constraints on exchange or (3) heterozygous for the multiply inverted chromosome TM2. The results show that the centromere, the telomeres, the heterochromatin and the euchromatic-heterochromatic junction do not serve as elements that respond to genic determinants of the regional distribution of exchanges. Instead, the results suggest that there are several elements sparsely distributed in the X chromosome euchromatin. Together with the controlling system affected by recombination-defective meiotic mutations, these elements specify the regional distribution of exchanges. The results also demonstrate that the alteration in the distribution of crossovers caused by inversion heterozygosity (the interchromosomal effect) results from the response of a normal controlling system to an overall increase in the frequency of crossing over, rather than from a disruption of the system of regional constraints on exchange that is disrupted by meiotic mutations. The mechanisms by which regional constraints on exchange might be established are discussed, as is the possible evolutionary significance of this system.