Evidence for negative interference: Clustering of crossovers close to the am locus in Neurospora crassa among am recombinants

In response to a conflict between two mapping studies in the predicted orie ntation of the allele map with respect to the centromere, Fincham proposed that recombination events at the Neurospora am locus rarely have an associa ted crossover. Fincham considered that the elevated levels of crossing over between flanking markers in am recombinants resulted from negative interfe rence, an increased probability of a nearby second event, and on this basis predicted a clustering of crossing over near am in these recombinants. In this article we reevaluate the data from three mapping studies of the am lo cus and report molecular evidence that shows crossovers to be clustered imm ediately proximal to am in am recombinants.