INDUCED CHROMOSOMAL ABERRATIONS IN ANIMALS 1195 



Serebrovsky (118), Dubinin (38, 39), Levit (58), and Shapiro (121) 

 have described the origin of several allelomorphs of the sex-linked reces- 

 sive genes scute and yellow associated with various chromosome abnor- 

 malities, such as inversions and translocations. Unfortunately, in 

 none of these cases is it convincingly proved that the mutations appeared 

 in the immediate proximity to the loci of the breaks. 



Changes in the genes located in the duplicating fragments of chromo- 

 somes constitute a somewhat different phenomenon. In the offspring of 

 wild-type flies treated with X-rays individuals may be found that carry 

 duplications for certain sections of the chromosomes. The duplicating 

 fragments, according to their origin, should carry the wild-type allelo- 

 morph of the genes involved. It is found, however, that in many cases 

 the genes located in duplications behave as though they had mutated 

 (Dobzhansky and Sturtevant, 37). Thus, a duplication was found 

 which should have included the wild-type allelomorphs of the sex-linked 

 recessives rudimentary and forked. Nevertheless, males having a normal 

 A"^ carrying rudimentary or forked, and having the duplication, show 

 quite clearly the effects of rudimentary and forked, respectively, in their 

 phenotypes. Likewise, rudimentary and forked manifest themselves 

 in females having two normal X's with these genes and the duplication. 

 The wild-type allelomorphs of rudimentary and forked are, however, 

 known to be almost completely dominant over one or even two doses 

 of the corresponding recessives. The behavior of these wild-type allelo- 

 morphs when located in the duplicating fragments, indicates, then, that 

 their functioning is somewhat changed by the occurrence of breakages 

 in their vicinity (cf. Dubinin and Sidorov, 41). An alternative explana- 

 tion of this fact may be that the change in the behavior of these genes 

 is due to a disturbance of the normal genie balance produced by the 

 duplications. 



Why should chromosome breakages so frequently be associated with 

 mutations in the neighboring genes? Several possible explanations of 

 this fact may be suggested. Perhaps the simplest one was advanced by 

 Bridges (Morgan, Bridges, Sturtevant, 71). Chromosome breakages 

 may be provoked by a destruction of certain genes in the chromosome. 

 The chromosome is, so to say, weakened in this place, its continuity 

 may easily be disrupted, and the breakage is accomplished. The 

 "mutations" at the breakage points are, then, deficiencies, losses, of 

 genes. The application of this explanation to most of the known cases of 

 "mutations" associated with breakages meets with no insuperable 

 difficulty. There is nothing surprising in this, since the same "explana- 

 tion" may be applied with equal ease to most of the spontaneous and 

 induced gene mutations as well (Serebrovsky, 117). The difficulty 

 comes, however, as soon as one tries to adduce specific evidence in favor 

 of this hypothesis. The known deficiencies possess certain character- 



