448 RADIATION BIOLOGY 



but were manifestations of the genes of this region in general. Hence 

 these characteristics became transferred in position when a part of the 

 latter did even when the above specialized genes were not included. 

 Smaller heterochromatic regions, showing most of the same peculiarities 

 but to a more limited degree, were found to be present just proximal to 

 the telomeres, and a part of the effects, still less well developed, were 

 found to exist in scattered interstitial positions as well. All this work 

 reciuired the production of structural changes by radiation. For most 

 of the points in this and the preceding paragraph the reader may be 

 referred to Muller (1938, 1944), Muller and Gershenson (1935), MuUer 

 and Prokofyeva (1935a), Muller, Prokofyeva-Belgovskaya, and Raff el 

 (1938) and Prokofyeva-Belgovskaya (1935, 1938, 1939). 



It was mainly through observations on the mode of transmission of 

 chromosomes changed by radiation, and on the phenotypic effects of 

 different combinations of the parts of these chromosomes, that the princi- 

 ples were worked out which govern the manner and speed of elimination, 

 retention, or multiplication of different types of structural changes in 

 populations. In this way it was possible for the present writer (1940b) 

 to make deductions concerning the role of different types of chromosome 

 alterations in evolution. The conclusions thus arrived at were corrobo- 

 rated by observations of other investigators, who had studied the dis- 

 tribution of chromosome differences among natural populations of the 

 same species, and among different races, subspecies, and species. 



24-2. Field of Gene Properties and Gene Evolution. Study of the somatic 

 effects produced in Drosophila by the addition or subtraction of given 

 chromosome pieces, made possible by the use of structural changes 

 obtained from irradiation experiments especially designed for the finding 

 of aberrations appropriate for this purpose, has resulted in a considerable 

 extension of knowledge concerning genie balance and related matters. 

 For example, the subtraction of a given portion of a chromosome from 

 the diploid set of chromosomes was found, in general, to cause a good 

 deal more developmental and physiological disturbance ihan the addition 

 of the same portion to it, in correspondence with the fact that the sub- 

 traction constitutes a greater relative change than the addition. Simi- 

 larly, the amount of disturbance caused by a given addition or subtraction 

 is smaller, the larger the number of complete chromosome sets (i.e. the 

 degree of "ploidy") present in the individuals being compared.'" 



1" It may well be because of this principle that so many dominant genetic abnormali- 

 ties, involving complexes of characteristics, have been observed in the first generation 

 of offspring of irradiated salmon spermatozoa, in the work of Foster, Donaldson, ei al. 

 (1949 and personal communication). There is reason to believe these fishes to be of 

 polyploid origin. Most of the abnormalities would in that case be caused by extra 

 or missing whole chromosomes or large chromosome pieces, which would have been 

 lethal to ordinary diploids, and the apparent contradiction between these results 

 and those on most organisms would thus be reconciled. 



