Genes — Nature and Consequence 



449 



amino acid may be the one which is normally 

 incorporated at that position in the cistronic 

 product. So, mutants which make wrong 

 messenger RNA may still form the correct 

 protein product, if the additional, com- 

 pensating error is made of having sRNA 

 carry a specific wrong amino acid. Through 

 these and other studies in phage we can ex- 

 pect to learn a great deal about the sequence 

 of nucleotides in DNA strands, the nature 

 of the genetic code and of mutation. Further 

 advances are expected from our increasing 

 biochemical capabilities to determine nucleo- 

 tide sequence in biologically made RNA and 

 to synthesize specific sequences of ribonucleo- 

 tides. 



In order to study mutation, it has always 

 been necessary in the past to observe a 

 change in the phenotype, that is, some mor- 

 phological, physiological, or biochemical 

 change in a trait. Since additions, subtrac- 

 tions, or shifts of chromosomal material can 

 be observed directly under the microscope, 

 without any attempt being made to deter- 

 mine whether an extrachromosomal trait is 

 involved, you might at first think such 

 changes are not changes in phenotype but 

 only changes in genotype. But the chromo- 

 some is surely just as much a part of the 

 phenotype as is some nongenic portion of the 

 organism. For example, the kappa particles 

 (themselves genie) are properly described as 

 being part of the Paramecium's phenotype 

 (and also as part of its genotype). One can 

 speak of the change in nuclear morphology 

 (hence phenotype) which occurs when cells 

 become polyploid or polytene (such changes 

 having a genie basis). One can discuss 

 changes in chromosomal phenotype during 

 different stages of mitosis (even though there 

 may be no gene change involved). It is 

 clear, then, that the genotype refers to all 

 the genes present, while the phenotype refers 

 to all the genie and nongenic traits of the 

 system. Of course, when one is working at 

 the macroscopic level the phenotype cannot 



include chromosomal or other genie traits, 

 and deals only with the consequences of the 

 interactions between genes and their en- 

 vironment. 



The phrase, a novel phenotype based upon 

 genie change, partially defines what we have 

 previously called a mutant (see p. 403). 

 The word "novel" requires some additional 

 consideration. It would have been entirely 

 correct to consider the first case of segrega- 

 tion as being a mutation, since it certainly 

 was a novel change in the genetic material, 

 one that had never before been recognized. 

 When, however, other gene pairs were 

 studied, it was found that segregation was 

 not a novelty after all, but the rule for paired 

 nuclear genes. We, therefore, include segre- 

 gation as a means of genetic recombination, 

 not of mutation. Similarly, genetic trans- 

 formation was first considered to be a rare 

 genetic change, and so was referred to as 

 mutation. But once a variety of species ex- 

 hibited this phenomenon, and once trans- 

 formation was recognized as being a fre- 

 quent event within certain species, it became 

 clear that, usually, the process is better con- 

 sidered as a mechanism for genetic recom- 

 bination. Recall also the discussion of 

 Modulator and Dissociation. Although the 

 position effects these genes apparently cause 

 were not considered mutations, the move- 

 ments of Modulator and Dissociation were. 

 More and more cases of the latter type of 

 change are being discovered. Are we still 

 justified in considering these as being muta- 

 tional? We will probably soon consider 

 cases like these to be examples of another 

 mechanism for genetic recombination, at 

 least in certain organisms. Finally, you may 

 recall that the integration and deintegration 

 of F were classified as genetic recombina- 

 tions, and not as mutations. You may have 

 been surprised at this at the time, but this 

 interpretation was given in the light of knowl- 

 edge (which had not yet been presented) that 

 other types of episomes were known. In this 



