same chromosome (Belling, 1927, 1933). Aside from the fact that this 

 hypothesis (Figure 6-10) fails to fit the data on pairs of adjacent chias- 

 mata, it also demands that duplication take place at or shortly after 

 the time of pairing, and it is difficult to see how, in its present form, 

 it can fit the evidence of the multistranded nature of the chromosome. 

 Also, if it is evoked to explain crossing over as well, it fails to account 

 for three- and four-strand double crossovers. 



(b) 



Figure 6-10. Schematic Representation of Chiasmata Formation Accord- 

 ing to the Duplication Theory Proposed by Belling. 



Classical Theory 



In the classical view proposed by Karl Sax in 1932, chiasmata result 

 from alternate opening out between sister (equational) and homologous 

 (reductional) strands with the kinetic loops being always reductional 

 (Figure 6-9 (c)). On this hypothesis, chiasmata do not represent the 

 result of crossing over, but crossing over may result from breakage and 

 rejoining at points of overlap. In this view also, reduction in the number 

 of chiasmata should be correlated with increase in crossing over since 

 each crossover represents ehmination of a chiasma. 



Differential Coniraction Theory 



Tension set up by differential contraction after pairing may cause 

 breaks at overlaps. This hypothesis is based on measurements which 

 indicate that contraction is greater in paired regions than in those not 



154 / CHAPTER 6 



