The Decrease of Crossing-Over Observed in Translocations, and Its Probable Explanation

Abstract

In simple translocations a fragment of one chromosome is attached to another, which remains intact. In reciprocal translocations two or more chromosomes exchange fragments. The transposed fragments usually include an end of one of the original chromosomes; but a middle section of one chromosome may be attached to another. Attachment may be terminal or subterminal. Reduction of frequency of crossing over is usually involved. The data indicate that the pairing of chromosomes, a prerequisite for crossing over, is caused by attraction of homologous loci (genes) in gametogenesis. Translocations produce compound chromosomes, parts of which are homologous to parts of two or more, chromosomes of the complex. A conflict of the attraction forces of its homologues prevents the compound chromosome from pairing. The reduction of frequency of crossing over is unavoidable under these conditions. This agrees with cytological studies of synapsis. The phenomena described in this article suggest a method for studying experimentally the action of the attraction forces.