1176 BIOLOGICAL EFFECTS OF RADIATION 



sterile due to the absence of the F-chromosome. This indicates that the 

 fragmentation of the X-chromosome by X-rays is produced in spermatids 

 or spermatozoa, but not in the spermatogonia or spermatocytes. 



Patterson (100, 101, 102, 103, 106, 107) showed that, owing to the 

 fact that chromosomes are already split in some of the spermatozoa at 

 the time of treatment, mosaic individuals occur in the progeny of X-ray- 

 treated flies. Such mosaics carry in a half of their bodies two normal 

 X-chromosomes, and in the other half one complete X, and another X, 

 a fragment of which is lost. If the normal X carries sex-linked recessive 

 genes and the treated X-chromosome is wild-type, such mosaic individuals 

 are readily recognizable. Patterson found that if a small section of the 

 X-chromosome is lost, the mosaic fly is completely female, but if the 

 deficiency is sufficiently long the mosaic becomes a gynandromorph, 

 the part of the body carrying the deficiency being male. 



Sturtevant (131, 133) proved that the majority of the so-called 

 "factors suppressing crossing over" in the chromosomes in which they 

 lie are actually inversions of sections of these chromosomes. Inversions 

 with which Sturtevant has worked were either found in nature or arose 

 spontaneously. Muller (72), Serebrovsky and his coworkers (120), 

 Serebrovsky (118), Oliver (90) and others found that inversions arise 

 frequently as a result of X-ray treatment. Inversions are detected 

 through a reduction of crossing over produced by them in the chromo- 

 somes affected. The laboriousness of this method is, no doubt, responsi- 

 ble for the fact that up to now we possess no systematic data on the 

 frecjuency of the induced inversions. 



THE MODE OF ORIGIN OF CHROMOSOMAL REARRANGEMENTS 



A number of investigators have obtained data relating to the problem 

 of the mechanism of the origin of the spontaneous and the induced 

 chromosome rearrangements. An extensive study on the relation 

 between the intensity of the X-ray treatment and the frequency of induced 

 chromosome rearrangements, and that of the induced mutations was 

 made in Drosophila melanog aster by Oliver (87, 89, 90). Oliver applied 

 five different dosages of X-rays {h to fie), which can be arranged in a 

 series in which every following member is twice as great as the preceding 

 one. The minimum dosage {t\) was equal to about 385 r-units. The 

 results of one of Oliver's experiments, in which only sex-linked lethals 

 and only those chromosomal rearrangements which are associated with 

 lethals were detected, are summarized in Table 2. 



The results of this and of other experiments show that the frequency 

 of both induced mutations and chromosome rearrangements is directly 

 proportional to the amount of treatment. The possibility that higher 

 dosages produce a somewhat disproportionally great effect on chromo- 

 somal rearrangements does not seem, however, excluded. 



