THE MECHANISM OF CROSSING-OVER 



II 



HERMANN J. MULLER 

 Rice Institute 



IV. The Manner of Occurrence of Crossing-over 

 A. Interference 



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 inter- 

 change. The question* suggested themselves, for ex- 

 ample, what was the total frequency of crossing-over, did 

 any factors separate more often than they remained to- 

 gether, 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 con- 

 nected 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 under- 

 went 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 

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