Crossing over near the Centromere of Chromosome 3 in Drosophila Melanogaster Females.

HE basic approach used in studying the mechanism of crossing over has been Tto alter crossover frequencies by manipulating extrinsic or intrinsic factors in an attempt to correlate the primary effect of the manipulation with the observed genetic results. These experiments have provided much data on the phenomenon of crossing over but little information on its basic mechanism. This is because very little is known of the primary effects of intrinsic alterations such as chromosome rearrangements or of extrinsic conditions, both of which may result in a spectrum of physiological changes. One result common to all treatments that induce changes in crossover frequencies is that crossing over is invariably increased in regions adjacent to or spanning a centromere. Thus, radiation (MULLER 1925), temperature (PLOUGH 1917), chemicals (BROWNING 1949; SUZUKI 1963a), and chromosome aberrations (see SCHULTZ and REDFIELD 1951 for a review) all increase crossing over in centromeric regions. The experiments reported here were carried out to determine: ( 1 ) whether the different conditions which result in increased crossing over are acting on the exchange process in different ways that lead to a similar end result or whether they all affect a common mechanism; and (2) whether all regions near the centromere are equally sensitive to the induction of increases in crossing over as opposed to a regional variation in response to the treatments.