1268 BIOLOGICAL EFFECTS OF RADIATION 



eight variant types (in addition to certain "monstrous forms"). Six of 

 these were found to be due to chromosomal aberrations. One (" awned ") 

 was considered "an undoubted case of locus mutation," and another 

 ("squarehead") was considered possibly due to gene mutation. The 

 eight variants all arose in the progeny from ears X-rayed at meiosis 

 and later; a larger group from treatments apphed at an earlier stage of 

 development yielded no variant types. Sapehin (36) has briefly reported 

 the occurrence of numerous variants in the progeny of common wheat, 

 of which the great majority were aberrant types, more or less sterile, 

 but among which some apparent mutants of practical interest were 

 included. The genetic analysis of these variants has not yet been 

 reported. 



Mutations have been induced in maize by X-ray treatment of pollen, 

 young embryos, or seeds (45, 45a). This plant is less favorable material 

 than barley for quantitative studies of mutation frequency, but because 

 of its unique combination of genetic and cytological advantages it is 

 much more favorable for the genetic analysis of the induced mutations. 

 The treatments are applied to parent stocks differing in genes marking 

 all 10 chromosomes. These are crossed and the Fi plants are self- 

 fertilized. The mutants segregate in F^ progenies segregating also for 

 the chromosome markers, and thus their linkage relations are indicated 

 at once. The presence of the marker genes also permits the recognition 

 in i^i and F^ of various types of chromosomal aberration occurring 

 in the same progenies. These include deficiency, inversion, segmental 

 interchange, and various modifications and combinations of these 

 phenomena. The chromosomal effects of X-ray treatment in maize 

 are discussed in Paper XLI. About 40 mutations affecting seed and 

 seedling characters were found in experiments thus far reported. All 

 of the mutations were recessive. They show normal linkage relations, 

 and their occurrence is not correlated with that of the gross chromosomal 

 alterations causing partial sterility. 



WHAT RADIATIONS ARE EFFECTIVE? 



The X-rays found to induce mutation in the experiments reviewed 

 above were relatively "soft" or low-frequency radiations, in most cases 

 those emitted at 50 to 100 k v. p. The results secured with this radiation 

 were in general similar to those secured with gamma rays of radium, 

 which correspond to extremely hard X-rays, of higher frequency than 

 any obtainable with ordinary X-ray equipment. The so-called "grenz 

 rays," which are very soft X-rays generated at 10 kv. p. and lower, induce 

 mutation similarly (43, 51r). Apparently X-rays of any degree of 

 hardness obtainable with present-day equipment are capable of inducing 

 mutation. 



