258 RADIATION BIOLOGY 



which have boon exposed to ultraviolet, r.sing a stock containing a 

 spcci.-i! ^■ chromosome (y''Y') marked with the normal allele of achaeto, 

 he matccl irradiated males to achacte females, 'rerminal l«»ss of th(> nor- 

 mal allele of acliaet(>, with retention of the remainder of the special Y 

 chromosome, would yield fertile achaete males in the Fi population, 

 l-'rom a total of H),;i01) h\ males, 23 sterile achaete exceptions were found. 

 Such males, however, result from a loss of the paternal X chromosome, or 

 of all or part of both arms of the y-'-Y^ chromosome, losses which could 

 arise through lagging of the chromosomes in division, or by breakage 

 followed by fusion of broken ends to give acentric and dicentric portions 

 having a low survival probability. Breakage of the y^-Y^ chromosome, 

 with loss of the y^ region (which also includes the normal allele of achaete) 

 and foUow-ed by healing, did not occur. The fertile achaete exceptions, 

 two in number, were no more frequent than in the control population. 



Experiments with Plants. In many species of plants the pollen grain 

 may be effectively treated with ultraviolet radiation. It is therefore 

 feasible to make somewhat simpler tests of the genetic effects of the treat- 

 ment than can be made in the experiments with Drosophila. The indirect 

 analysis required by the polar cap technique is avoided, and the difficul- 

 ties from internal filtration, while serious, are not nearly so great as in 

 the irradiation of the sperm within the body of the adult fly. In the cul- 

 tures grown from seeds produced by the use of the irradiated pollen (which 

 we may, for convenience, refer to as the Fi cultures), each plant provides 

 the material for testing the effects of the treatment on one irradiated 

 gamete. Dominant effects of chromosome or gene changes induced by 

 the treatment may be observed in the Fi plants, and each F2 culture pro- 

 duced by self-fertilization of one of these plants shows segregation for any 

 haplo-viable recessive alteration induced in the gamete tested. 



The results reported by Noethling and Stubbe (1934) clearly demon- 

 strated the effectiveness of ultraviolet in inducing mutations. These 

 were detected in segregating F2 populations of Aniirrhinum. 



A similar increase in the frecjuency of point mutations was found in 

 maize by Stadler and Sprague (193(m), together with evidence of certain 

 chromosomal effects of the treatment and further indications of differ- 

 ences in the genetic action of ultraviolet radiation and X rays. The 

 mutations identified were only those affecting seed and seedling charac- 

 ters. Progenies representing various doses of unfiltered ultraviolet radia- 

 tion yielded 31 mutations from 830 gametes tested, and control progenies 

 yielded 6 mutations from 557 gametes. 



This increase in mutation rate, while clearly significant, is not large. 

 But among the 31 mutants detected (of which 9 would have been expected 

 without irradiation), there were two cases in which 2 unlinked mutants 

 occurred in a single F2 progeny, and one case w^ith 3 unlinked mutants in a 

 single progeny. These represent cases of two or three presumably unre- 



