The Mechanism of Crossing-Over

As soon as it seemed probable that the factors were linked in line, and that the crossing-over was the actual method of interchange, it became of interest to discover and to analyze the precise mode of incidence of the interchange. The questions suggested themselves, for example, what was the total frequency of crossing-over, did any factors separate more often than they remained together, how often did crossing-over occur at two points simultaneously, and was there any tendency, in such cases, for the two points of crossing-over to be a definite distance apart, or in definite positions, etc. For answers to these questions might throw light on the mechanism of crossing-over, what cytological phenomena it was connected with, and what stage in synapsis it occurred at. With these points in view the author calculated the linkage relations that would result on several possible schemes of interchange. The simplest possibility was that the chromosomes always twisted in loops of fixed length, though not of fixed position, and always underwent breakage, with recombination of homologous strands (i. e., crossed-over in the technical sense), at each place that the strands crossed one another. In such a case there would always be a definite distance between one point of crossing-over and another; moreover, all factors which were separated by a distance great enough for double crossing-over to occur between them, i. e., by the length of at least one loop, must always have either double or single (or multiple) crossing-over between them. Sturtevant's data, however, showed that this was