CYTOGENETIC CORRELATIONS AND CROSSING OVER 109 



Slowly this chiasmatype theory became generally accepted, and until 

 recently most cytogeneticists agreed that every chiasma represented a 

 genetic crossover, and that there were no chiasmata without crossing 

 over. The observation by Cooper of chiasmata in the male Drosophila, 

 in which there is no genetic crossing over, has cast some doubt upon the 

 generality of Janssens' theory. On the other hand, there is fairly good 

 quantitative evidence indicating that the frequency of chiasmata parallels 

 the frequency of crossing over found in maize. Interlocking rings and 

 other special configurations found with aberrant chromosome pairing also 

 support the chiasmatype theory. The present situation is thus an un- 

 satisfactory one: no fully acceptable hypothesis has been proposed to 

 account for the occurrence of chiasmata. 



Chiasma Interference 



The frequency of recombination between closely linked markers is 

 often found to be lower than expected on the basis of linkage with more 

 distant markers. This phenomenon has been attributed to interference 

 of one chiasma in the formation of a neighboring second chiasma. In 

 actuality, the term interference has only a formal significance, since 

 the mechanism is not known. With very closely linked markers, an in- 

 crease in the frequency of recombination above that expected has been 

 observed and given the logically correct but clumsy title of negative 

 interference. 



Tetrad analysis is particularly well suited for the quantitative estima- 

 tion of interference, and studies with several microorganisms are being 

 carried out to investigate the process of interference in relation to the 

 mechanism of crossing over as a whole. 



Chromatid interference and Sister-Strand Exchange 



If an investigation is made of the recombination of linked markers 

 situated some convenient distance, say 5 to 10 map units apart, one finds 

 that exchanges are always reciprocal, with the recovery of all the input 



break nor the translocated segment of the parental X chromosomes while +B flies 

 had both (adopted from C. Stern, 1931, Biol. Zbi, 51:547). 



(b). Maize experiment in which chromosome 9 was labeled both genetically and 

 cytologically. Noncrossover progeny had the same chromosomes as did the parents, 

 while C Wx progeny contained the translocation but not the knob, and c wx progeny 

 contained one knobbed chromosome but no translocation. Thus the exchange must 

 have occurred between C and Wx as diagrammed (adapted from Creighton and 

 McClintock, 1931, Proc. Natl. Acad. Sci. Wash. 17:492). 



