PROBLEMS OF MEASUREMENT OF MUTATION RATES 57 



Ste7i}: In the snapdragon, Antirrhinum, there are some reports on 

 simultaneous mutation of two alleles (38). 



Auerbach: Spontaneous? 



Stern: Yes. Some of these cases concern highly mutable genes. 



Auerbach: Because in induced mutation, there are several people 

 who make the statement that chromosome breaks (which, of course, 

 are something different) tend to occur in homologous spots in the two 

 chromosomes, in genetic treatment. 



Neel: Kim, if I understand correctly, your message is that in muta- 

 tion, no matter how we measure it and no matter in what form, the im- 

 portant thing is the cell division; that if we put rates in terms of cell 

 divisions, our mutation rates will be very similar from one form to 

 another. The apparently higher rate in man is related to the greater 

 number of cell divisions that intervene between conception and func- 

 tional sperm. Is this correct? 



Atwood: As far as it goes. I meant to leave the impression that there 

 is no particular reason to think that because we have differences in 

 measured mutation rates among organisms, the rates of the elementary 

 processes involved are correspondingly different. Instead, our estimates 

 of the rates have different meanings; they are not really comparable 

 w^ithout further assumptions. The question of relation to cell division 

 is only one aspect of this. There are many others. 



Neel: Cell division is the aspect that has come up repeatedly. What 

 others? 



Atwood: The question of direction of the mutation, whether it is 

 forward or reverse. The size and environment of the mutable units. 

 Selection effects at the cell level may play a role in some instances in 

 metazoans, and then the various mistakes that can be made in 

 measuring rates where difficult assumptions are required, as is often 

 the case in man. 



Neel: You are maintaining then that mutation, whatever it is, occurs 

 at about the same rate in all manner of organisms? 



Atwood: To put it more cautiously, we have no particular reason to 

 think that it is not occurring at spontaneous rates having similar range. 



Magni: On the other hand, the range of mutation frequency in the 

 same organism is so wide that it is difficult to say whether the muta- 

 tion rate is different from one organism to the other. 



Benzer: Yes, that's right. 



Neel: Per life cycle, or per cell division within the organism? I am 

 not clear what our point of reference is in this attempt to say that all 

 organisms have about the same mutation rate. 



