An inferred chiasma map of Drosophila melanogaster.

The chiasma map of D. melanogaster is inferred from the genetic map, giving correspondences between physical and genetic locations. Crossing-over is reduced near the centromere and telomeres. If chiasmata occur preferentially near the telomere they must be distal to genetic loci. A precept of Bridges and Morgan is discussed, which Drosophila genetics neglected but chromosome mapping in other organisms should follow.