The genetic effects of four recombination-defective meiotic mutants in D. melanogaster on recombination, segregation and the relationship between the two have been examined. The results suggest the following. (1) The anomalous meiotic segregation observed in females carrying recombination-defective meiotic mutants is a normal consequence of the reduction in exchange; each recombination-defective mutant can, therefore, be defined by a single lesion in the control of recombination. (2) Of the operations used to date to characterize this lesion, the most informative is whether the decrease in recombination is uniform along the chromosome arm or nonuniform; in particular, if the formation of recombinants is visualized as a two-step process consisting of the establishment of possible exchange points (exchange preconditions) followed by exchange itself, then mutants that uniformly decrease crossing over involve defects in the second step while mutants that result in a nonuniform decrease involve defects in the establishment of exchange preconditions. (3) Of the fourteen loci identified by recombination-defective meiotic mutants, only one (with two alleles) is involved in exchange itself; the others all reduce recombination most drastically in distal regions, suggesting that the establishment of exchange preconditions involves polar processes. (4) A very general description of the polar establishment of exchange preconditions is presented; this description has the property that if a precondition meiotic mutant affects interference, the coefficient of coincidence will be increased in proportion to the decrease in recombination which is what is observed for all recombination-defective meiotic mutants studied to date.
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