NATURE OF THE GENETIC EFFECTS 451 



regions. Nevertheless, the problem of the physicochemical nature of 

 position effects is still an unsettled one. 



Another series of studies in which chromosome changes induced by 

 radiation have been utilized for the light they throw on gene properties 

 has dealt with the number of genetically separable positions of breakage 

 which exist within a given minute chromosome region, measurable in 

 salivary preparations (Muller and Prokofyeva, 1935b). In this work a 

 series of structural changes, all selected, through their position effects, to 

 have one chromosome break located very near a certain gene ("scute"), 

 were genetically cross-tested with one another by special methods which 

 made it possible to determine the positional order of the breaks in the 

 chromosome, from left to right. This involved getting recombinations 

 between the different cases of structural change, having the left-hand 

 portion of one changed chromosome (A) extending up to its break in this 

 region, together with the right-hand portion of another (B) extending 

 rightwards from its break, to discover whether the AB combination was 

 lethal or exhibited any other phenotypic abnormality indicative of a 

 deficiency. The complementary BA combinations, having the left part 

 of B with the right part of A, were obtained and examined similarly. 

 When AB proved deficient break A was shown to be to the left of break 

 B, while when BA was deficient break B was to the left of A; but when 

 neither recombination behaved as a deficient one, it could be concluded 

 that the breaks were at identical positions, in the sense that no genetic 

 material having a detectable influence on the organism lay between them. 



It turned out that some dozen cases of breakage in the given region 

 involved only four positions of breakage, as thus determined, and from 

 certain additional evidence just one more possible position of breakage 

 was deducible. Each two breakages thereby defined, lying in consecutive 

 positions, enclosed between them a gene with distinguishable effects. 

 The evidence thus indicated that the chromosomes become broken only 

 in certain discrete positions, between genes, and that the genes are to be 

 regarded as distinct entities. It may be recalled that evidence for the 

 same conclusion was also given by the finding that, in most organisms, 

 which show little or no position effect, structural changes are very seldom 

 accompanied by lethal effects or other phenotypic abnormalities. 



A study somewhat similar to the above in principle although not in 

 technique has been made in maize by McClintock (1938b, 1941, 1944). 

 Here chromosomes structurally altered by X rays were used which have 

 a sequence of breakage-fusion-bridge cycles, resulting (when homozygous 

 or when in combination with certain other chromosomes which had also 

 been structurally altered by radiation) in plants and parts of plants 

 wholly deficient for a small chromosome region. Two different chromo- 

 somes (5 and 9) were used in different series of experiments. The limits 



