AKE GUSTAFSSON 



further worked out and definitely proved by Mac key^. He stated the 

 fonowing values of significance for a difference in mutating 



between mustards and X-rays P = • 04 

 ,, „ ,, neutrons P= 0-001 



„ „ «2p P = 0-002 



Similarly D'Amato^ found that acridin derivatives altered the mutation 

 process, so that albina mutants are not formed, although xantha and viridis 

 mutants are frequent. We are safe in concluding that we now possess the means 

 of changing the spectrum of the mutation process (Kaplan^, Muller^^). In 

 the cases mentioned this is the more remarkable since chlorophyll mutations 

 arise after changes in a great many gene loci, say roughly 250-300. Of these 

 about one half presumably cause albinas (125-150) ; somewhat less than 

 one half cause viridis types (100-125) ; less than a tenth, xanthas and albo- 

 virides (10-15 respectively) ; the remaining very rare types of mutants 

 comprise according to this method of computation approximately 15-20 loci. 



Table I 



An indication that individual loci may be changed by different physio- 

 logical treatments before irradiation was afforded in 1940 and 1947 by the 

 induction of the rare so-called alboxantha mutants. These preferably arise 

 when germinating seeds are irradiated. A similar experiment was then 

 made by Anderson^ (cf. Ehrenberg et al^). He found that in Escherichia 

 coli a special mutation was depending for its origin on the oxygen pressure 

 at the time of irradiation. 



In recent studies Ehrenberg and Andersson^** as well as Ehrenberg and 

 Saeland^^ detected that these rare types of chlorophyll mutations to a 

 certain extent depend on the character of the radiation employed, as visual- 

 ized in Table I. 



In the case of these chlorophyll mutants we deal with lethal or semi- 

 lethal changes of the genotype. The ultimate aim of our experimental 

 work, however, is to induce specific mutations leading to changes of viable 

 or even high-productive character. In fact, we now know in barley how 

 to increase its stiffness of straw — a property of supreme agricultural value — 

 by means of mutation. All so-called erectoides mutations so far studied 

 possess an extremely stiff straw. The anatomical and physiological back- 

 ground of this peculiar characteristic has been worked out by Wettstein^^. 

 The fifteen or twenty loci, giving erectoid mutations, behave in a similar 

 fashion. Recently Ehrenberg and Nybom^' discovered that there is an 



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