CYTOGENETIC CORRELATIONS AND CROSSING OVER 111 



chromosome duplication. If this were correct, it would seem that only 

 the new strands could be involved in an exchange. Consequently, with 

 the demonstration that all four strands take part, Belling' s hypothesis 

 was dropped. However, if one invokes the occurrence of sister-strand 

 crossing over, then Belling s hypothesis can be reconsidered, as shown in 

 Figure 4.9. There is at present no critical evidence either establishing 

 or excluding the occurrence of sister-strand exchange at meiosis. In a 

 recent re-evaluation of crossing over based upon all the available genetic 

 data in Drosophila melanogaster, Weinstein has concluded that either 

 sister-strand crossing over does not occur or, if it does occur, that it does 

 not interfere with nonsister exchanges between homologous chromosomes. 

 This conclusion does not permit a decision with respect to the Belling 

 hypothesis. 



As the result of new studies of chromosome duplication, utilizing radio- 

 active tracers to distinguish between parental and newly formed strands, 

 it is becoming possible to reinvestigate crossing-over mechanisms at the 

 molecular level. New genetic data, obtained by mapping very short 

 regions, indicate that some exchanges may occur by a process very differ- 

 ent from that envisaged from studies of recombination over larger inter- 

 vals. These problems will be reconsidered in Chapter 7, after presenting 

 some recent information on chromosome duplication and fine-structure 

 mapping. 



Physiological Influences upon Crossing Over 



Before closing the discussion of classical studies of crossing over, it 

 should be noted that crossing over is neither an invariant process occur- 

 ring at all meioses nor a random process insensitive to environmental 

 influences. No crossing over occurs at meiosis in the Drosophila male 

 which is XY, nor in the silkworm female, which is XO. In general, in 

 those organisms in which differences in crossover frequency have been 

 observed between the sexes, it is the XY or XO sex in which crossing over 

 is reduced or absent. Such factors as age, nutritional condition, and 

 temperature alter crossover frequencies considerably in Drosophila, but 

 not all regions are uniformly affected. Those near the centromere are 

 most susceptible. 



These varied facts, not at all understood, are of interest for two rea- 

 sons: Any theory of crossing over must eventually explain them; and, 

 conversely, they may provide some clues or leads in the investigation of 

 crossing-over mechanisms. 



In summary, this chapter has cited some of the classical evidence 

 which established the chromosomal location of genetic determinants, 



