1278 BIOLOGICAL EFFECTS OF RADIATION 



are fully viable in the gametophyte and thus are transmitted in full 

 proportion to the next sporophyte generation. If the mutations are due 

 to deficiencies which are without injurious effect in the haploid generation, 

 there should also be intermediate variations due to deficiencies which 

 though not lethal are still not fully viable. Among these there might be 

 losses large enough to permit cytological identification. Stadler (47) has 

 reported that there are many such intermediate variations, and several 

 of these show the loss of chromosome segments long enough for cyto- 

 logical detection. For example, in one case described in detail, an appar- 

 ent recessive mutation of the gene R" was shown to be due to the loss of a 

 chromosome segment including the locus of this gene. The deficiency 

 in this case is transmitted with reduced viability by female gametophytes 

 but is not transmitted by male gametophytes. In an induced recessive 

 mutation of the gene Yg2, viable in both male and female gametophytes, 

 Creighton (9) has shown that a small deficiency occurred which reduced 

 the size of a "knob" known to be located very close to the locus of the 

 gene Yg^. No deficiency could be seen in the chromonema, and evidence 

 from other cases showed that the locus of the gene is not within the knob. 

 The presumption is that the deficiency included the locus of Ygi and a 

 part of the knob, but that the section of chromonema lost is too short for 

 cytological detection. 



The question of the physical nature of induced mutation has not yet 

 received a satisfactory answer. It is clear that many of the variations 

 identified by their genetic behavior as gene mutations are due to mechani- 

 cal alterations analogous to the grosser chromosomal aberrations. But 

 no mechanical explanation of reversible mutations has thus far been 

 found, and the most plausible assumption is that these mutations are 

 intragenic. Possibly germinal alterations of both types are included 

 among the induced mutations; possibly mechanical alterations not yet 

 understood are responsible for the reversible variations. This question 

 must be left for future investigation. 



REFERENCES 



1. Avery, A. G., and A. F. Blakeslee. Radium experiments with Datura. IV. 

 (Abst.) Anat. Rec. 41: 99. 1928. 



2. Baur, E. Untersuchungen iiber das Wesen, die Entstehung und die Vererbung 

 von Rassenunterschiedungen bei Antirrhinum majus. Bibliog. Genet. 4: 1-170. 

 1924. 



3. Beadle, G. W. A gene for sticky chromosomes in Zea mays. Zeitsch. Indukt. 

 Abstamm. und Vererbungsl. 63: 195-217. 1932. 



4. Blakeslee, A. F. The chromosomal constitution of Nubbin, a compound 

 (2n - 1) type in Datura. Proc. Nation. Acad. Sci. 13: 79-85. 1927. 



5. Blakeslee, A. F. Nubbin, a compound chromosomal type in Datura. Ann. 

 New York Acad. Sci. 30: 1-29. 1927. 



6. Blakeslee, A. F., A. D. Bergner, S. Satina, A. G. Avery, J. L. Cartledge, 

 and J. S. Potter. Analysis of Datura stramonium plants grown from seed 

 exposed to x-rays. (Abgt.) Anat. Rec. 44: 281. 1929. 



