Studies on the Mechanism of Crossing over.

N E of the major objections to the torsion theory of crossing over as pro-0 posed by DARLINGTON (1935) is that it is difficult to comprehend why a break in one chromatid should cause a break to occur in exactly the same position in a chromatid from the homologous chromosome. This objection holds for any mechanism which postulates that crossing over occurs after the duplication of the chromosome into chromatids. However, as will be discussed, the localization of the crossover at identical loci offers no difficulty if it is postulated that crossing over is the result of an exchange between the new chromatids during the process of their formation (BELLING 1931). Such crossing over, of course, will give rise to only the 2-strand type of multiple cross-overs, and the hypothesis requires exchanges between sister chromatids to explain the 3-strand and 4-strand multiple crossovers as was pointed out by LINDEGREN and LINDEGREN (1937), In the face of strong objections to assuming the occurrence of sister-strand crossing over, BELLING modified his original theory, thereby weakening it considerably (BELLING 1933). However, in view of evidence which has recently been presented supporting crossing over between sister strands (SCHWARTZ 1953), we should reconsider the hypothesis that crossing over between homologous chromosomes involves only the newly formed or daughter chromatids. On such a hypothesis, two crossovers between homologous chromosomes will give rise to a 2-strand type of double exchange. A double exchange associated with a sister-strand crossover in one of the chromosomes, in the region between the two non-sister crossovers, will be converted into a 3-strand double. A sister-strand crossover in each of the homologous chromosomes will result in four single exchange chromatids, a 4-strand double. This is shown diagrammatically in figure 1. If the frequency of sister-strand crossing over is high, as was shown to be the case from the study of ring-rod heterozygotes in maize (SCHWARTZ 1953), and there is an equal probability of an even or odd number of sister-strand exchanges occurring in the region between the two non-sister-strand exchanges, the four double crossover classes would appear in a 1 : 1 : 1 : 1 ratio, that which is also expected (in the absence of chromatid interference) as a consequence of crossing over which involves exchanges between any of the chromatids of the homologous chromosomes. We are thus confronted' with the problem of distinguishing between these two hypotheses, each of which yields …