A more critical test of hypotheses of crossing over which involve sister-strand exchange.

rather novel mechanism for crossing over based on BELLING’S ideas A (1933)-was suggested by LINDEGREN and LINDEGREN (1937) and its implications were thoroughly discussed by SCHWARTZ ( 1954). It was proposed that crossing over of the usual type (between nonsister strands) takes place between only two of the four chromatids, presumably the newly synthesized ones. In order to explain the occurrence of three-strand and four-strand double exchanges, it was assumed that sister-strand exchange also takes place. (Note that a sisterstrand exchange, by definition, occurs between a parental and a newly synthesized strand.) Irrespective of one’s intuitive feeling towards such a model of crossing over, it has the significant merit of being subject to experimental testing. SCHWARTZ ( 1954) designed an ingenious experiment in Drosophila melanogaster to test its validity for somatic crossing over. He reasoned that one would expect twin spots of yellow and singed tissue to be produced in y sn+/y+ sn flies when an exchange takes place between these genes and the centromere in normal free X chromosomes, but one would not expect such an exchange to produce twin spots if the two X chromosomes were attached and if exchange were limited to the newly synthesized chromosomes. The basis of his reasoning is shown in Figure 1. Under conventional mechanisms of crossing over, one would expect the frequency of twin spots to be the same in free X and in attached-X chromosomes if one assumes that reciprocal and nonreciprocal exchange in attached-X chromosomes, and type-I and type-2 disjunction in free X chromosomes, are equally likely. There is no reason to doubt this assumption. However, on the SCHWARTZ hypothesis, only reciprocal, nonsister, exchanges occur in attached-X choromosomes and these will not form twin spots unless accompanied by a sister exchange between them and the centromere. Therefore, according to his hypothesis, the frequency of twin spots will be lower in attached-X as compared with free X chromosomes, unless one makes the unlikely assumption that half of the time when a reciprocal exchange occurs, it is accompanied by a sister exchange (or an odd number of sister exchanges) in this specific place. The “unlikely assumption” is equivalent to assuming an infinite amount of sister exchange, half producing even and half producing odd-numbered crossovers.