268 CHROMOSOME ABERRATION PRODUCTION 



examinations are then made at appropriate intervals after treatment to 

 detect aberrations at the first postmeiotic mitosis in the microspore. 

 Two general categories of aberration types are noted: (a) chromatid 

 types, resulting from irradiation of chromosomes which are effectively 

 double in prophase, and (6) chromosome types, resulting from irradiation 

 of chromosomes which are effectively single in resting stage. In both in- 

 stances, radiation produces breaks in one or more of the six chromosomes 

 in the nucleus of a microspore, and the resulting broken ends may re- 

 main as such, undergo restitution, or rejoin with other broken ends to 

 produce aberrant configurations which are cytologically detectable. The 

 principal configuration types with which we shall be concerned in the 

 experimental results to be discussed in this paper may be designated as 

 chromosome interchanges. These rearrangements, observed in cells 

 examined 4-5 days after irradiation, are either dicentric or ring chromo- 

 somes and result from breaks in two separate, undivided chromosomes, 

 or chromosome arms, followed by the reunion of broken ends to give 

 one chromosome with two centromeres, or one continuous ring chromo- 

 some, plus an accompanying acentric fragment in each case. 



The essential features of the hypothesis developed by Sax, Lea, and 

 others to explain the production of chromosome aberrations in Trades- 

 cantia may be outlined as follows. Electromagnetic or particulate radi- 

 ations produce their effects as a consequence of the formation of ion 

 pairs within a chromosome during the passage through the chromosome 

 of either primary or secondary charged particles. Chemical changes re- 

 sulting from this direct ionization of the molecules composing the chromo- 

 some lead to the production of chromosome breaks. The resulting 

 broken ends may remain as such, yielding terminal deletions, or undergo 

 restitution, giving rise to apparently normal chromosomes again, or re- 

 join with other broken ends, producing cytologically visible aberrations. 

 The restitution and reunion processes are competitive and both space 

 and time factors are involved. Several ionizations (betw^een fifteen and 

 twenty) must occur within the chromosome to produce a break, and the 

 factor of major consequence in distinguishing the quantitative effects of 

 various radiations is the difference in ionization distribution along par- 

 ticle tracks. For certain radiations (for example, gamma rays and x- 

 rays) ionization distribution along the tracks (of secondary electrons) is 

 such that the probability of breakage of a chromosome by a traversing 

 particle is considerably less than 1 for most of the length of the track 

 except near the end. As a consequence, such radiations are relatively 

 inefficient in producing breaks, on the basis of total ionization produced 

 per track. Furthermore, breaks in two separate chromosomes (or chro- 

 mosome arms) are almost always produced by two separate particle 



