MUTAGENESIS 



83 



m 



r+m 



Figure 15. Decrease in the fraction of mottled plaques with 

 increasing mutational hits. Points indicated by different sym- 

 bols belong to independent experiments. 



m = frequency of mottled plaques per plaque forming phage, 

 r = frequency of r plaques per plaque forming phage. 



ments that we have done with hydroxylamine (26) which are concerned 

 with such mosaics. Hydroxylamine is a mutagen which acts preferen- 

 tially on cytosine bases. We have used it on phage T4 similar to the 

 work of Tessman and Vielmetter and Schuster with nitrous acids. But 

 we have also determined the ratio of mottled plaques versus the sum of 

 r and mottled plaques with respect to the frequency of the sums of r 

 and mottled plaques, r plaques represent the pure mutants while the 

 mottled plaques correspond to your mosaics (Fig. 15). I don't want 

 to go into the reason why I have chosen this particular abscissa; it has 

 to do with other lethal properties of hydroxylamine. We find a lot of 

 scattered points but we can put approximately a straight line through 

 them. 



