1272 BIOLOGICAL EFFECTS OF RADIATION 



may be determined both genetically and cytologically. Since the 

 deficiency is lethal to the gametophyte, deficient gametes are not pro- 

 duced by the deficient plant; its progeny therefore is entirely normal. 

 Other frequent chromosomal effects of irradiation are reciprocal trans- 

 location and inversion. While these chromosomal rearrangements do 

 not necessarily involve any loss of chromosomal material, they result 

 in the production of gametophytes characterized by deficiency or by 

 both deficiency and duplication. The deficient gametophytes are 

 aborted in these cases also. 



But in the polyploid species, since the gametophyte has more than a 

 single set of chromosomes and genes, deficiency is not regularly lethal. 

 The loss of a chromosome segment will be lethal only if the portion lost 

 contains some essential gene or genes which are not duplicated in the 

 other chromosome groups. Other losses will permit gametophyte 

 development and the production of deficient gametes. These deficiencies 

 will be transmitted and any phenotypic effects they may have will 

 therefore be heritable. The various chromosomal effects of irradiation, 

 which in nonpolyploid species result in partial sterility, w^ill now con- 

 stitute a source of induced germinal variations, resulting not from 

 modified genes but from the deficiency or duplication of specific segments 

 of the chromosomes. Among the variations of this class, those caused 

 by chromosomal alterations which have no effect on the transmission 

 of the gametes will be inherited as if due to a change within a gene. 



The results of the experiments with polyploid and nonpolyploid 

 species of Triticum and Avena are in harmony with these considerations. 

 The frequency of mutations affecting seedling characters was deter- 

 mined in four species of each genus (42). In the species with 7 pairs 

 of chromosomes mutations of this sort were frequent, as previously found 

 in Hordeum vulgare which also has 7 pairs of chromosomes. In species 

 with 14 pairs of chromosomes such mutations were much less frequent, 

 and in species with 21 pairs of chromosomes they were extremely rare. 

 Using the same species of Triticum, Tascher (see 46) found the frequency 

 of partial pollen sterility induced by X-ray treatment to be related 

 similarly to chromosome number. An X-ray treatment which resulted in 

 partial sterility in a large percentage of the plants treated in T. mono- 

 coccum (7 pairs) produced partial sterility in a much smaller proportion 

 of the treated plants of T. durum (14 pairs) and in very few cases in T. 

 vulgare (21 pairs). Partial sterility induced similarly in Zea Mays has 

 been found to be due chiefly to deficiency and translocation (45). Pre- 

 sumably the same chromosomal modifications are induced in the Triticum 

 species, both polyploid and nonpolyploid. In the latter the deficient 

 germ cells are aborted ; in the former many of them survive and transmit 

 nonlethal deficiencies and duplications to the progeny. The very low 

 frequency of induced partial sterility in the polyploid species indicates 



