INDUCED MUTATIONS IN PLANTS 1273 



that a surprisingly large proportion of the induced deficiencies must be 

 viable. 



The fact that induced variations inherited as typical mendelian char- 

 acters are found in common wheat, therefore, is not proof of the occur- 

 rence of gene mutation in the polyploid species. If even large deficiencies 

 fail to cause pollen abortion, it may be expected that among the minor 

 deficiencies and duplications some will have no appreciable effect on 

 gametic survival and will thus be transmitted as mendelian factors. 



The phenotypic effects which may be produced by such chromosomal 

 irregularities are not limited to the expression of genie unbalance result- 

 ing from the loss or addition of genes en masse. In addition to these 

 (which may themselves be indistinguishable from the effects of changes in 

 individual genes), the loss of a chromosome segment in a polyploid may, 

 by "uncovering" a specific recessive, lead to the expression of a known 

 mutant gene apparently not present in the stock before treatment. For 

 example, in wheat, if the recessive gene a ("awned") is present in two of 

 the three constituent genomes, but the corresponding dominant A 

 (awnless) is homozygous in the other genome, the strain will be a true- 

 breeding awnless wheat. If a chromosome segment including A is lost 

 as a result of irradiation and the deficiency has no effect on viability, the 

 deficient gametes will be aa and the nondeficient gametes aaA. The 

 genetic effects of this change will be precisely the same as if the mutation 

 had been a change from A to a. The fact that gametes deficient for an 

 entire chromosome may be transmitted through both male and female 

 gametophytes in hexaploid species as Avena and Triticum, as shown by 

 Kihara (26), Huskins (24, 25), and Nishiyama (33, 34), makes it possible 

 that wholly viable sectional deficiencies may be common in the progenies 

 of irradiated plants of these species. Similarly Clausen and Good- 

 speed (8) and Lammerts (27) have shown that in Nicotiana Tabacum and 

 N. rustica gametes lacking an entire chromosome may be transmitted, at 

 least through the female gametophyte. 



Goodspeed (17) has emphasized the possible chromosomal basis of 

 apparent mutations induced by radiation in N. Tabacum. In a variant 

 characterized by pink flower color, which behaved genetically as if due to 

 gene mutation, Goodspeed (18, 19) has demonstrated cytologically the 

 presence of attached chromosome fragments. The chromosomal aberra- 

 tions responsible for the occurrence of these fragments may have been 

 causally connected with the occurrence of the germinal variation. 



THE PHYSICAL NATURE OF TRANSMUTATION BY X-RAYS 



The basic problem in the study of the genetic effects of irradiation is 

 the physical nature of the changes induced in the genes and chromosomes. 

 This is fundamental not only to problems of induced mutation, but to the 



