7 



Chromosome duplication and 

 genetic recombination 



An important goal of modern genetics is the description of events at 

 the molecular level to account for the recombination of linked genes. 

 The partial chiasmatype theory of crossing over, discussed in Chapter 4, 

 provided a formal model of chromatid exchange at the four-strand stage, 

 in keeping with the principal observations of classical cytogenetics. 

 With the advent of microbial genetics, it became feasible to study re- 

 combination at a level of resolution not possible previously. The fine- 

 structure mapping which resulted led to findings at variance with some of 

 the basic tenets of the classical theory. 



Individual deviations from the partial chiasmatype theory were first 

 observed in Drosophila and subsequently in Neiiwspora, but for a time it 

 seemed that only phage recombination operated on a different principle. 

 Subsequently, more and more of the presumed special features of the 

 phage system were regularly observed in Neurospora and in other organ- 

 isms with a conventional meiosis. Recombination was shown to occur 

 within a functional genetic unit, negative interference was demonstrated 

 within very short map intervals, and aberrant segregation ratios of alleles 

 were observed at meiosis. 



These findings have compelled a revision of classical concepts of re- 

 combination. A detailed analysis of the molecular events underlying 

 chromosome duplication and exchange at meiosis is needed for their un- 

 derstanding. Such an analysis is just beginning; the models of recombi- 

 nation thus far proposed are based primarily upon the available genetic 

 data because there is not yet sufficient information available from 

 chromosome studies. Nonetheless, questions of chromosome organiza- 

 tion and duplication are under active investigation, and, because the 

 results are of central importance for genetic analysis, we shall review 



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