294 



C. LEVINTHAL 



"Are the genetic exchanges reciprocal as one expects for simple cases of 

 crossing over, or must one look for an alternative mechanism more intimately 

 connected with the mode of reproduction of the virus?" Of the various pheno- 

 mena which have been studied there are three which have yielded most of our 

 meager insight into this problem. 



Fig. 4. Three schemes by which recombinants could be formed by the interaction of 

 two parental genetic structures: (a) the classical crossing-over model, which yields two 

 reciprocal recombinants; (b) the partial-replica or copy-choice model, which forms one 

 recombinant and conserves the integrity of the parents; and (c) the partial-replica model, 

 in which all structures are considered as duplex and in which the recombinant contains 

 the heterozygous overlap region in the vicinity of the switch. 



A. Reciprocal Recombinants 



As was mentioned previously, the equality of two reciprocal recombinants 

 in any cross is a direct consequence of the lack of selection among the various 

 genetic types. However, in higher organisms, wherever it is possible to per- 

 form the required experiments, it is found that the reciprocal recombinants 

 are formed together in the same elementary mating act; largely for this 

 reason, the elementary process in higher organisms is thought of as a crossing- 

 over event. It is, unfortunately, much more difficult to determine whether 

 the reciprocal recombinants arise in the same act in phage than it is, for 

 example, in Neurospora, where two pairs of chromosomes enter the mating 

 and all the products of the exchanges are held together in a single membrane 



