INDUCED CHROMOSOMAL ABERRATIONS IN ANIMALS 1185 



part of the chromosome in Fig. 4). This part of the chromosome is 

 called the inert region. Little crossing over takes place within this 

 region (for its genetic length is extremely small), no mutations except at 

 the locus of bobbed have been observed, and individuals in which this 

 region is deficient survive at least as heterozygotes, while deficiencies for 

 sections of similar lengths in other chromosomes are invariably lethal. 

 Nevertheless, the part of the chromosomes occupied by the inert region 

 appears quite normal cytologically, and translocations and inversions 

 are known to involve break- 

 ages of the chromosome in 

 this region. Since bobbed is 

 the only known sex-linked 

 gene having an allelomorph 

 in the F-chromosome, and 

 since the F-chromosome is 

 also composed of genetically 

 inert material, the inert 

 region of the X is probably 

 homologous to a section of 

 the F-chromosome. The 

 function of the inert region 

 in the germ plasm is not 

 definitely known. It is this 

 region of the X- which pairs 

 with the F- chromosome at 



meiosis, and this fact makes p,^. 4.-Cytological maps of the chromosomes of 



it tempting to suppose that DrosophUa melanogaster, showing the approximate 



its function is to insure l^^f'^Ve^L^fr.ir^^^^ 



(sj). llie inert region of the A -chromosome repre- 

 normal pairing and disjunc- sented by the stippled portion of the rod-shaped 

 tion of thp X-V nnir of rhra chromosomes. The longer V-shaped chromosomes 

 lion 01 tne A-r pair Ol cnro- (igf^^ ^^^ ^^e third chromosomes; the shorter V- 



mOSOmeS in male gametOgen- shaped chromosomes (right) are the second chromo- 

 esis. No inert regions are «o°^es;tte smallest pair, the fourth chromosomes. 



SO far known in the autosomes of DrosophUa, but the data now available 

 are insufficient to exclude the possibility that short genetically 

 inert regions may be present there. (This, indeed, has been proved 

 to be the case by Heitz (49, 50), who discovered that the middle 

 sections of the second and third chromosomes, adjacent to the spindle 

 attachments, behave in prophases differently from the remaining parts 

 of these chromosomes, and similarly to the inert region of the X-chromo- 

 some. These sections are, then, probably inert in the same sense as 

 the proximal third of the Z-chromosome is inert. Whether or not the 

 loci of some of the known genes are located in these inert regions (as 

 suggested by our cytological maps, Fig. 4) remains to be seen. If this 

 were true, the "inertness" of these regions would not be disproved. 



