Genetic and Non-Genetic Variation 287 



mitochondria of the yeast cells. It is not sure whether the 

 bleaching effect of streptomycin on the chloroplasts of algae 

 belongs to the same category, because there are no genetical 

 data on this (Provasoli, Hutner and Pintner, 1951). 



Finally, with nuclear mutations the pattern of specificity 

 disappears. True, there is a pattern of "relative specificity" 

 of mutagens both in Neurospora (Kolmark, 1956) and in 

 bacteria (cf. Demerec et al., 1956). However, this specificity, 

 which can be very striking (Table II) is found only with 



Table II 



The relative specificity of 6 mutagens on the double mutant 

 INOSITOLLESS (No. 37401), ADENiNELESS (No. 38701). The mutagens are 

 compared under "optimal conditions"* 



Mutations per 10 ^ conidia Proportion 



inos^ ad^ ad^ linos'^ 



(1) CB 1528t 11-3 17-4 1-5 



(2) Ultraviolet light 7-1 3-5 0-5 



(3) Diethylsulphate 4-3 16-8 4 



(4) Dimethylsulphate 3-4 64 19 



(5) CB 1506J 0-3 51-0 190 



(6) Diepoxybutane 0-2 89-0 445 



* Data from Kolmark (1956, and unpublished). 



t Ethyl methanesulphonate (CHs-CHa-O -SOa-CHs) 



X Chloroethyl methanesulphonate (Cl-CHa-CHo-O -SOa CHs) 



reverse mutations. The induction of forward mutations by 

 mutagens does not show the same degree of specificity. The 

 explanation for this difference may be quite simple. Most 

 mutagens act by destroying or inactivating the genetical 

 material. There are so many ways in which a gene may be 

 damaged or inactivated, that a striking specificity is hardly 

 to be expected. However, a few genes may be changed so 

 slightly by the mutagenic treatment, that the change may be 

 reversible. It would, however, require a specific mechanism 

 to repair a damaged gene. Therefore it is not surprising that 

 the pattern of specificity of mutagenic treatment shows up in 

 back mutations but not in forward mutations. 



There is, however, one special kind of specificity in the 

 response of the nucleus to mutagens. In the Neurospora 



