1276 BIOLOGICAL EFFECTS OF RADIATION 



have been found in plants, and some of these have phenotypic effects on 

 the sporophyte when heterozygous. It therefore seems probable that 

 the absence of induced dominant mutations in plants is due to the fact 

 that all induced dominant variations are lethal to the gametophyte. 



But dominant genes of regular genetic behavior and viability, such as 

 the genes for plant colors and morphological characteristics which 

 distinguish agronomic varieties, although sought in very extensive experi- 

 ments with plants, have never been found to occur as a result of irradia- 

 tion. On the contrary, viable mutations of mutant genes to the dominant 

 wild type have been found to be induced with appreciable frequency in 

 Drosophila. 



Moreover, recessive mutation of various genes in plants also seems to 

 be independent of radiation. The effect of the treatment on the fre- 

 quency of mutation of a specific gene cannot be determined without 

 information on the natural or spontaneous rate of mutation of the gene 

 concerned. This has been determined for seven genes affecting endosperm 

 characters in maize (44, 46). Radiation experiments indicate that only 

 one of these genes is appreciably affected in mutation rate by heavy 

 treatment with X-rays. Since these genes are an unselected sample, 

 this result indicates that the proportion of recessive gene mutations 

 affected by radiation may be small. 



B. The parallel response of deficiencies and mutations to variations of 

 dosage is shown by the experiments of Stadler (41, 45a) and Goodsell (15). 

 The frequency of mutation increases as a linear function of the total 

 intensity of radiation applied; that of deficiencies producing mosaic 

 endosperm varies in the same way with the radiation intensity. Wide 

 variations of temperature during irradiation are without effect on the 

 frequency of induced mutation, and this is true also for endosperm 

 deficiencies. 



A more striking parallel is the similar reaction of the two types of 

 genetic variation to metabolic activity of the cells irradiated. Mutation 

 is induced by irradiation of wholly dormant tissue, but the frequency of 

 mutation per unit of radiation intensity is much lower than in active 

 tissue (43). The chromosomal aberrations resulting in partial steriUty 

 also occur in cells irradiated while dormant, and their frequency also is 

 greatly reduced, approximately in the same proportion as that of muta- 

 tion (46). 



It is interesting to note also that mutations similar to those induced 

 by irradiation seem to occur in connection with chromosomal aberrations 

 due to other causes. Sprague (37) has found that exposure of the maize 

 ear shortly after fertilization to the effects of an electromagnetic field 

 results in the occurrence of endosperm mosaics and other chromosomal 

 disturbances. In the progeny of plants from seeds given this treatment a 

 few .seedling mutations were found. Beadle (3) has described a strain 

 of maize characterized by high frequency of translocation and other 



