CHROMOSOMAL MUTATIONS 



fects may simulate genes at a considerable distance, he considered that 

 the genie properties must actually reside in small segments of the chro- 

 mosome, within which change of order is mutation. 



With regard to this, Goldsclimidt * said, "It might be stated first that 

 there has been much misunderstanding of our conclusions. There is, of 

 course, no doubt that the chromosome has a serial structure and that 

 localized changes of this structure, the mutant loci, can be located by the 

 cross-over method. There is no doubt either that these localized condi- 

 tions of change can be handled descriptively as separate units, the mutant 

 locus or gene, and that for all descriptive purposes the extrapolation can 

 be made that at the normal locus a normal gene exists. Further, there can 

 be no doubt that almost all genetical facts can be described in terms of 

 corpuscular genes, and that a geneticist who is not interested in the ques- 

 tion of what a gene is may work successfully all his life long without 

 questioning the theory of the corpuscular gene. In the same way, ... a 

 chemist can describe and handle almost all the content of chemistry with 

 valences represented as one or more dashes between atoms. But when 

 he wants to know what valence is, he has to use the tool of quantum 

 mechanics which the ordinary chemist does not need in his work. Simi- 

 larly, the concept of the corpuscular gene comes under scrutiny only 

 when the problem of the nature of the gene and the explanation of muta- 

 tion and position effect is attacked." 



Functional Organization of the Chromosome. Goldschmidt, then, 

 rejected the idea of the corpuscular gene, with inactive spaces between 

 the genes. Rather, he conceived of the chromosome as a more unified 

 structure, a chemical continuum, in which the functional units ( genes, in 

 the traditional terminology) may be very small segments for some re- 

 actions, or larger segments for other reactions. Nor is there any reason 

 why the functional segments for different genie reactions should not over- 

 lap. Finally, the whole chromosome may function as a unit in some in- 

 stances, although this would be difficult to prove. There is some evidence 

 that this may be the case for the sex-determining action of the X chromo- 

 somes. This may be restated in terms of the salivary gland chromosomes. 

 Kodani has shown that the salivary gland chromosomes are basically 

 tetrads. In each band the chromonemata are coiled, with 8 to 12 chromatic 

 hairs or bulbs radiating from these coils in the plane of the band ( Figure 

 86 ) . Interstitial chromatin is packed between the chromatic hairs to com- 

 plete the band. The interband spaces are greatly hydrated, and contain 

 so-called matrix protein so as to expand to the diameter of the bands. 

 A rearrangement of the chromatic hairs within a band would be undetect- 

 able by present means, and hence, if it should have a mutant effect, it 

 would be called a point mutation, although in fact it would be a position 

 effect. 



On the next level of function, a whole series of bands, corresponding 

 to a single chromomere of the leptotene chromosome, might act as a unit. 



' Goldschmidt, R. B., "Position Effect and the Theory of the Corpuscular Gene," 

 Experentia, 2, 24 (inclusive pp., 1-40), 1946. 



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