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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1935 



W 



which the genetic experiments lie it does not make the slightest dif- 

 ference whether the gene is a hypothetical unit or whether the gene 

 is a material particle. In either case the unit is associated with a 

 specific chromosome and can be localized there by purely genetic 

 analysis. Hence, if the gene is a material unit, it is a piece of a 

 chromosome ; if it is a fictitious unit, it must be referred to a definite 



location in a chromosome — the 

 same place as on the other hypoth- 

 esis. Therefore, it makes no dif- 

 ference in the actual work in genet- 

 ics which point of view is taken. 

 Between the characters that are 

 ased by the geneticist and the genes 

 that the theory postulates lies the 

 whole field of embryonic develop- 

 ment, where the properties implicit 

 in the genes become explicit in the 

 protoplasm of the cells. Here we 

 appear to approach a physiologi- 

 cal problem, but one that is new 

 and strange to the classical physi- 

 ology of the schools. 



We ascribe certain general prop- 

 erties to the genes, in part from 

 genetic evidence and in part from 

 microscopical observations. These 

 properties we may next consider. 

 Since chromosomes divide in 

 such a way that the line of genes 

 is sj)lit (each daughter chromo- 



ciency. (b) Two chromosomes of Indian g^j^^^ receiving CXactlv half of 

 corn, one having a deficiency near its . . ,. . 



the origmal line) we can scarcely 

 avoid the inference that the genes 

 divide into exactly equal parts; 

 but just how this takes place is 

 not known. The analogy of cell 

 division creates a presumption 

 that the gene divides in the same 

 way, but we should not forget that the relatively gross process in- 

 volved in cell division may seem quite inadequate to cover the refined 

 separation of the gene into equal halves. As we do not know of 

 any comparable division phenomena in organic molecules, we must 

 also be careful in ascribing a simple molecular constitution to the 

 gene. On the other hand, the elaborate chains of molecules built up 

 in organic material may give us, some day, a better opportunity to 



Figure 3. — (a) Two conjugating chiomo 

 somes of Indian corn (after McClintocli) 

 One chromosome has a terminal defi 



middle. When these two chromosomes 

 conjugate there is a loop in the longer 

 chiomosome opposite the deficiency in the 

 other one. (c) Two chromosomes of In- 

 dian corn, one having a long inverted 

 region. When they conjugate they come 

 together as shown in the figure to the 

 right, nice genes coming together. 



