MUTAGENESIS 145 



Does it really represent a return to the original configuration at the 

 same site? To test for suppressor mutation, 3'ou back-cross to the wild 

 type and see if you can get segregation of the individual mutants. Feyn- 

 man found this for some apparent revertants, but his revertants were 

 deliberately chosen because they looked false (by phenotype) to begin 

 with. When I did this experiment using only revertants which looked 

 genuine, it failed. 



Another is the following one. If it has really returned to the original 

 wild type, it should have exactly the same mutation frequency at the 

 same site as before. This is a very stringent test of the idea that this 

 is a true reverse mutation. This experiment has been done for nine 

 different revertants, and all but one gave exactly the same mutation 

 rate, at the same point as before. Without this test, I would consider 

 the measurement of an apparent reverse rate as unreliable. This is 

 particularly pertinent to experiments on induction of reversion by 

 mutagens. Short of a proper demonstration that you return back to 

 the original type^ you do not know whether you are working on a 

 reversible transition or inducing mutations at some suppressor site. 



This experiment, when applied to a mutant with a very high rever- 

 sion rate — one of these rates excluded from the forward class — still 

 gave the same result, so that genetic transitions at a very high rate, 

 which are truly reversible, are possible. 



It is quite frequent, and this is especially characteristic of the 

 spontaneous hot spots, that the mutations arising at the same point 

 have different reversion rates. One case has been studied in some de- 

 tail, at which three different reversion rates were found. 



This experiment of forward, reverse, and forward again, has been 

 done with all three types, and in every case we got back the same 

 forward mutation frequency at the site. 



As I pointed out, to do this experiment, the revertants which I 

 picked were chosen because they really looked like standard type. To 

 do his experiment, Feynman deliberately picked ones which looked as 

 though they were not a complete return to the standard type, and in 

 his cases he found that the suppressor mutations were at other 

 points, 



Auerbach: In Neurospora, Giles (29) got partial reversions which 

 seemed to have occurred at the same site, and yet were not full 

 reversions. Of course, the structural analysis can't be so fine with 

 Neurospora. 



Benzer: One of the revertants we studied appeared to be not quite 

 identical to the original standard type but had a very slightly mutant 



