MUTATIONS 



0.6 



CRUDE MUTATION RATE 



Figure 1. Distribution of 50 bacterial and 39 human 

 crude spontaneous mutation rates. Solid bars = bac- 

 terial; open bars = human. The rates are not ex- 

 pressed in comparable units; human rates are per 

 human generation and bacterial rates per bacterial 

 generation, or approximate equivalent. 



Auerbach: Did you include only those which were obtained by a 

 screening technique or are so-called forward mutations also included? 



Atwood: In microorganisms most of these are back mutations. Those 

 that are not back mutations are mostly phage-resistance. 



Demerec: It makes a great deal of difference what kind of muta- 

 tions you are considering. Back mutations are changes, presumably, 

 in one specific site, whereas forward mutations may be changes in any 

 one of a great many sites. 



Atwood: Yes. The apparent rates in man are higher than in microbes 

 by about a factor of 10^, and a number of things may contribute to 

 this factor. One is the one you just mentioned, that if you have a 



