The Journal of Heredity 



age first, followed by adhesion oi broken 

 ends. It was early evident thai by the 

 use of such rearranged chromosomes ad- 

 ditional proof of the physical validity of 

 the linkage maps could be obtained, and 

 this was done (Muller and Painter). 

 Furthermore, it has been possible to 

 throw light on problems of crossing over, 

 as in the demonstration (Muller, Stone, 

 and ( MTermann > that to whatever posi- 

 tion the centromere is moved, it causes 

 a strong inhibition of crossing over, the 

 strenth of which gradually diminishes 

 with distance. Moreover, the same 

 proves to be true of any point of dis- 

 continuity in pairing, caused by hetero- 

 zygosity in regard to a structural change. 

 Such studies on crossing over, and on 

 the pairing forces that affect segregation, 

 are still capable of considerable exten- 

 sion. 



We must remember, in speaking of the 

 centromere and other apparently distinc- 

 tive chromosome parts, that we have no 

 right to infer that they are autonomous, 

 locally determined structures, dependent 

 on the genes of the regions in which 

 thev are seen to lie, before observations 

 have been made that show the effects of 

 removing or displacing those regions. 

 Therefore, it has in the main been neces- 

 sary to wait for the study of induced 

 inversions, deletions and translocations 

 of chromosomes, before the inference 

 could be secure that the centromere is, 

 in most instances, such an autonomous 

 organelle, dependent upon a gene or 

 genes in the immediate neighborhood 

 (but not in all instances in the neighbor- 

 hood, as Rhoades has recentlv shown in 

 a special strain of maize). Similarly, it 

 has been possible to show (despite some 

 contrary claims, the validity or invalidi- 

 tv of which cannot be discussed here) 

 that the free end of the chromosome, or 

 telomere, constitutes in much material a 

 locally determined distinctive structure. 

 By a combined genetic and cvtological 

 analysis of various cases of breakage 

 and rearrangement of parts, it was found 

 that there are distinctive, largely locally 

 determined, regions of the chromosomes, 

 usually most markedly developed near 

 the centromeres, which we at first called 



"inactive" but which are now usually 

 referred to as "heterochromatic." These 

 were also found independently in pure- 

 ly cytological studies by lleitz. It would 

 be fascinating to enter here into a dis- 

 cussion of the remarkable peculiarities 

 which the cytogenetic studies have shown 

 these regions to have — the evidence of 

 repetition of more or less similar parts, 

 of a tendency to conjugation between 

 the differently placed parts, of distinc- 

 tive cytological appearance correlated 

 with whether or not such conjugation 

 occurs, of inordinately high tendency to 

 structural change, of strong influence of 

 certain of their genes upon segregation, 

 etc.. — and then to go on to discuss hy- 

 potheses of their evolutionary origin and 

 their functions. This would unfortunate- 

 ly take us too far afield. We must, how- 

 ever, insist upon one point — as it is not 

 yet generally enough recognized. — name- 

 ly, that the evidence is very strong that 

 what, in the Drosophila chromosome as 

 seen at mitosis, is called "the heterochro- 

 matic region," is simply a large tempo- 

 rary body of accessory, non-genic nuc- 

 leoprotein, produced under the influence 

 of one or two particular genes from 

 among the dozen or more that constitute 

 the whole heterochromatic region, as de- 

 tected by genetic analysis and by the 

 chromosome as seen at the resting stage 

 (as in the salivary gland). And it is 

 not these conspicuous non-genic blocks 

 which are responsible for the other 

 known peculiarities of the heterochro- 

 matin, above mentioned — the function 

 of the blocks is still undetermined. In 

 other words, the so-called "heterochro- 

 matin" with which the cytologist deals 

 in studying mitotic chromosomes is a 

 ouite different thing from, although in 

 the neighborhood of. the heterochro- 

 matin proper having the above described 

 complex of properties. Moreover, it has 

 been possible to show (Sutton-Gersh in 

 collaboration with the author, unpub- 

 lished) that the conspicuous nucleoli 

 often associated with the heterochromat- 

 in are produced under the influence of 

 still other autonomous genes in it. that 

 are senara f e from those for the mitotical- 

 ly visible blocks. 



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