1192 BIOLOGICAL EFFECTS OF RADIATION 



Stern found that most individuals of this type actually had the predicted 

 type of chromosomes. 



Creighton and McClintock's (21) methods of investigation, as well 

 as their findings, are in principle identical with those of Stern. These 

 results leave no reasonable doubt that crossing over actually occurs by 

 segmental interchange between homologous chromosomes. Any theory 

 not assuming such an interchange has to resort to the utterly unjustifiable 

 expedient of supposing that a chromosome may be broken in one place 

 and may grow a hook in another place every time a phenotype of the fly 

 shows certain mutant characteristics. Although such an attempt to 

 evade the consequences of Stern's and McClintock's work has been 

 actually published, it is hardly necessary to discuss it here. 



GENE CHANGES ASSOCIATED WITH CHROMOSOME BREAKAGES 



Translocations and inversions modify the order of the genes in the 

 chromosomes, produce new linkage relations, and alter the mode of 

 inheritance of the genetic factors located in the chromosomes involved. 

 They need not, however, affect either the number or the kind of the 

 genetic factors, and therefore individuals heterozygous or homozygous 

 for these chromosome rearrangements should be normal in appearance 

 and in viabihty. These theoretical expectations, however, are not always 

 realized, at least not in Drosophila. Translocations and inversions are 

 frequently lethal in homozygotes and some of them produce visible 

 abnormalities in the phenotype. 



The first translocation discovered in Drosophila (Bridges 16; Bridges 

 and Morgan 19) produces in heterozygous condition a dominant effect 

 on the eye color and is lethal when homozygous. Muller (72) found that 

 a majority of induced translocations are in viable in homozygous con- 

 dition. Muller 's findings were corroborated by observations of Muller 

 and Altenburg (78), Dobzhansky (25, 26, 29, 30), Dobzhansky and 

 Sturtevant (36), Oliver (90), and of many others. Although some 

 translocations are viable and normal in appearance in homozygous 

 condition (Dobzhansky, 25, 29), this is, at least in Drosophila, the excep- 

 tion rather than the rule. Not infrequently translocations produce 

 visible external effects (Muller, 75; Burkart, 20). The most remarkable 

 fact concerning the lethal and the visible effects of chromosomal rear- 

 rangements is that these effects are, in many cases, due to the action of 

 factors located in such close proximity to the points at which the chromo- 

 somes were broken that they fail to separate from the latter by crossing 

 over. It appears that breakages of chromosomes are frequently associ- 

 ated with a peculiar kind of mutation in the genes lying immediately 

 adjacent to the loci of the breaks (cf. Patterson, Stone, Bedichek, and 

 Suche, 109). An analysis of this correlation between "mutations" and 



