RECOMBINATION ANALYSIS IN MICROBIAL SYSTEMS 73 



past. Further, it seems unlikely that a clear understanding of recom- 

 bination mechanism will be possible in systems of equal parental con- 

 tribution until a clear distinction is made between the two basic types 

 of recombination which are at the origin of a single type of selected 

 recombinant. 



How, finally, stands the problem of relating genetic maps to the 

 lengths of nucleotide sequences? Clearly, it will be at an impasse until 

 a verified theory of recombination at the molecular level is evolved. To- 

 day classical mapping procedures seem to pertain to the fulfillment of 

 the primary condition that must be met if crossing-over is to occur— 

 i.e., eflFective pairing— rather than to the process of crossing-over itself. 

 When we examine recombination in transformation, in which we are 

 certain that the hybridizing element is DNA, and where— owing to the 

 very fragmentary nature of the "male" genome of the cross— selection 

 recovers only a single type of recombination event, the observed results 

 can be explained best in terms of a model in which the relation be- 

 tween the map distance and the recombination frequency is non-linear. 

 Further, when we examine data obtained by selection of rare recom- 

 bination events in Mendelian systems, we find, again, definite indica- 

 tions that recombination frequency may not be a simple function of dis- 

 tance between sites. It is time, therefore, to question the wisdom of 

 employing classical assumptions in mapping the fine genetic structure 

 of chromosomes. 



If this lengthy discussion has not left the impression that genetics 

 is a science of unifying principles, it is because the limitations of space 

 have confined us to preferential consideration of microbial systems. 

 However, manv of the newer features of chromosome behavior about 

 which this paper is written were foreshadowed in experiments with 

 maize and Drosophih. Such striking similarities of genetical behavior in 

 viruses, bacteria, fungi, and higher plants and animals we suppose to 

 be due to the common molecular structure of genetic material through- 

 out all living organisms on this earth. Accordingly, all aspects of recom- 

 bination must be due to the way in which DNA replicates and func- 

 tions. Even though remarkable progress has been made in understand- 

 ing the structure of DNA, its synthesis, and replication, we are still 

 very far from understanding recombination mechanisms in these terms. 

 Thus, while the proposed model is at the molecular scale, it is not a 

 molecular model. We can hope, however, that the formal genetic 

 description of recombination at the molecular scale will provide valu- 

 able parameters for the construction of a truly molecular theory of 

 recombination. 



References 



Avery, O. T., MacLeod, C. M., and McCarty, 1944. "Studies on the Chemical 

 Nature of the Substance Inducing Transformation of Pneumococcal Types. 



