1224 BIOUXilCAL EFFECTS OF RADIATION 



genes. It may be remarked (see Stadler, 166) that this is of particular 

 interest in connection with Hanson's (59) reversions at the Bar locus. 



Similarly for the brown locus, a series of dominant brown allelo- 

 morphs — all of them producing variegated eye colors — were detected in 

 progenies from X-ray experiments. These were then (46, 47, 48, 111, 

 149, 186, 188) found to be associated with chromosome rearrangements 

 which in all cases had one of the points of breakage near the brown locus. 

 Furthermore, in two of the cases tests were made (Schultz and Dob- 

 zhansky, 149) of the genes lying in the region of the other break, and 

 mutations were found to have occurred there also. It was also possible 

 to demonstrate that no simple occurrence of gene deficiency could account 

 for these results. This case, it should be noted, is complicated by the 

 variegation which is associated with all these mutations and whose 

 analysis is essential for the understanding of their nature. 



In the case of bobbed and of cubitus interruptus, the rearrangements 

 have been selected and then studied for mutations. Sivertzev-Dob- 

 zhansky and Dobzhansky (159) found that all of the five aberrations with 

 breakage's near the bobbed locus showed, under the proper circumstances, 

 mutations to bobbed. Similar cases have also been reported by Sidorov 

 (157), and by Stern (168a). Again there is an association between 

 breakage in a given region, and mutation. 



The most striking case of all, however, concerns the extensive series 

 of translocations involving the fourth chromosome, tested by Dubinin 

 and Sidorov (41). Out of 19 translocations, 10 exhibit an apparent 

 mutation to cubitus interruptus, when heterozygous for this recessive. 

 Dubinin and Sidorov further show that the translocations themselves 

 do not manifest cubitus interruptus under conditions when they might 

 be expected to, did they contain the gene mutation itself. The phe- 

 nomenon is then a decrease in the dominance of the normal allelomorph 

 of cubitus interruptus which is present in these 10 translocations. Also 

 in certain of their translocations, mutations are found at both points 

 concerned in the translocation. This behavior of cubitus interruptus 

 may be compared with Dobzhansky and Sturtevant's (34) data on the 

 decrease of dominance in a series of X-chromosome duplications. 



All of the cases discussed agree in the correlation of specific mutational 

 effects with breakages in specific regions. They do not, however, afford 

 as yet a demonstration of position effect comparable to that given in the 

 original case of Bar. In each case, specific objections may be raised which 

 prevent complete conviction. In the case of baroid, mutation alone may 

 account for the result; in the dominant eye colors, the variegation com- 

 plicates the issue; bobbed may be concerned with the general problem 

 of the inert region (Muller and Painter, 120; Dobzhansky, 32); and the 

 cubitus interruptus case, the most extensively tested, may merely involve 

 mutation to a different potency of wild-type allelomorph. Moreover, all 



