198 W. Hayes 



will be unable to express itself phenotypically until it has 

 become separated from the wild-type genes by nuclear and 

 cellular division. In this way, although bacteria are haploid 

 organisms, relationships of dominance and recessiveness can 

 come into play following mutation. Such relationships are 

 found, in a more orthodox way, in bacterial parasexual sexual 

 systems in which a fragment of the chromosome from a 

 donor cell is introduced into a recipient cell. This fertilized 

 cell becomes a partial heterozygote within which a process 

 of recombination occurs. It is not known for certain what 

 this process involves, but the evidence suggests that the 

 incoming fragment of donor chromosome and the chromosome 

 of the recipient cell pair together and then begin to replicate. 

 During this operation, a replica commenced on one of the pair 

 may suddenly switch to Topy the other, and thence switch 

 back to continue copying the chromosome on which it started. 

 In this way a completely new chromosome is produced which, 

 although basically that of the recipient cell, incorporates part, 

 or the whole, of the incoming donor fragment. From this 

 stage onwards it is likely that the recombinant chromosome 

 segregates into an autonomous cell in the same manner as a 

 mutant chromosome, so that something may be learned about 

 the phenotypic expression of mutations from the study of the 

 kinetics of segregation and expression in the parasexual 

 systems of bacteria. 



A direct approach to the kinetics of phenotypic expression 

 of mutations is a difficult matter from both the theoretical 

 and experimental points of view. In order to study the 

 sequence of events as a function of time, it is necessary to 

 have a high degree of synchrony among these events in the 

 population, and to be able to observe their progress from their 

 inception. By definition, such synchrony is impossible in the 

 case of spontaneous mutations which are rather rare events 

 whose occurrence is determined by chance. Moreover, muta- 

 tions can usually only be demonstrated by selective tech- 

 niques so that we can detect only those mutations which have 

 already become expressed at the time when the selective 



