Expanding genetic maps: reevaluation of the relationship between chiasmata and crossovers

The paradigm of a 1:1 relationship between cytological chiasmata and genetic crossovers is widely accepted (see Jones, 1987, for review). Extensive literature on meiosis will not be reviewed here, but John (1990) provides a good review of its various aspects. It was Darlington (1932, 1934), who compared chiasma frequencies with genetic map lengths in maize, both for individual chromosomes and the whole genome, and found a good correlation between the two estimators of recombination. With the recent explosion in genetic map construction in diverse organisms using DNA restriction fragment length polymorphisms (RFLPs), genetic maps based on RFLPs far exceed the length expected based on chiasma counts. Nilsson et al., (1993) summarized data from different crop plants documenting the discrepancy between the estimated number of crossovers from chiasma counts or genetic maps (Table 17.1). In every case, chiasma counts underestimated crossover frequencies calculated from recombination data.