Discussion 45 



the word "genetic". Then we can make the distinction that the genetic 

 change is controlled by the genotype. 



Pontecorvo: We have, of course, an operational test in some cases. In 

 Esch. coH we can use segregation and recombination, either by a sexual 

 process or by transduction. In other organisms, for instance ascomycetes, 

 we have the ordinary test of sexual reproduction and segregation as well 

 as "parasexual" segregation, etc. So we can unequivocally distinguish 

 in the majority of cases, even in micro-organisms, a genetic change from 

 an inheritable change which is not genetically determined. 



Davis: It seems to me that the science of genetics must be concerned 

 with all mechanisms of inheritance, and not simply those involving 

 chromosomal genes. Indeed, the term "gene" was surely derived from 

 "genetics" and not vice versa. I therefore wonder whether it might not 

 be useful to use the term "genetic" to include all mechanisms of in- 

 definitely transmitted inheritance, both chromosomal and non-chromo- 

 somal, and to use the term "genie" for chromosomal mechanisms. 



Kunicki-Goldfinger : One should be very careful when differentiating 

 between genie mutation and physiological, more or less stable, change, 

 especially if recombination analysis is not possible. 



In this connexion some phenomena may be pointed out which are 

 apparently due to mutation, but which are, in fact, caused by physio- 

 logical changes in bacterial cells. In an Esch. coli population only a very 

 small fraction of cells can grow in the presence of lithium chloride. Not 

 more than 1 per 100,000 cells is capable of forming a colony on media 

 containing lithium chloride. In the majority of strains these resistant 

 forms are not stable and their progeny are as susceptible as the parental 

 strain. Without analysis of population during growth the change may be 

 interpreted as being due to the selection of pre-existing mutants. In 

 reality it is caused by a physiological adaptation in a small fraction of 

 the heterogeneous population. 



The characteristic growth curve, which Prof. Cavalli-Sforza dis- 

 cussed, may also be due to selection of spontaneous mutants, or to 

 overgrowing of the culture by a new physiological variant induced by 

 the environmental conditions. This is the case in Brucella grown in syn- 

 thetic medium. A gro\\i:h curve with many peaks is then obtained. At 

 least some variants, whose growth resulted in the formation of additional 

 peaks, were shown to be of non-mutational origin. Some R-variants 

 could be obtained from homogeneous S-populations in conditions exclu- 

 ding cell multiplication. In this case the majority of cells were trans- 

 formed into a new type. If this change is not due to semi-stable physio- 

 logical adaptation, it may be caused by total mutation of almost the 

 whole population, induced by environmental factors, which seems to me 

 less probable. 



Hotchkiss: Prof. Davis has pointed out quite clearly what the con- 

 ceptual disagreement is. I suggest we turn more to the expermiental 

 inconsistencies. Sir Cyril has mentioned that cultures selected in low 

 concentrations of streptomycin would be resistant to high levels of 

 streptomycin. I know that in many organisms one may find streptomycin 

 resistance also: so I would like to know whether a low resistance is 



