128 Nature of the Genetic Material 



closer inquiry to be actually position effects of inversions, transloca- 

 tions, duplications, deficiencies; and new examples are found all 

 the time (see my example in I 3 B fc). This correction, linked with 

 the available improvement of technique or its absence, shows that 

 there can be no important difference between point mutation and 

 rearrangement except the means of recognition. In a former paper 

 (1944) I showed that approximately half of the mutants described 

 for D. melanogaster (which included multiple alleles) were already 

 known to be position effects. Thus we shall have to show first that 

 any type of mutant action known can be duplicated by proved 

 position effects. 



Before presenting this material we must return briefly to some 

 facts that have been mentioned repeatedly before. We defined the 

 position effect as the action of an intact locus in the more or less 

 distant neighborhood of a rearrangement break, as if it had mutated 

 to one or another of its known (or not yet known) alleles. A break 

 near the white locus produces a white phenotype, though it can be 

 proved that the white locus has not changed (is still the wild-type 

 allele of classic genetics). This position effect may be of two types. 

 It produces the likeness of a standard mutant if on the yonder side 

 of the break euchromatin is adjacent. (The reports on intercalary 

 heterochromatin are not yet decisive.) If, however, chromocentral 

 heterochromatin is adjacent, the mottled phenotype appears, which 

 has been already discussed. We mentioned also the different eu- 

 heterochromatin sequences which can result in mottling (see I 2 C d 

 hh). In the present example it is a mottled eye of red and white (and 

 also other colors) in different relative quantities and patterns. Some 

 geneticists have considered the latter type as something different, and 

 have tried to interpret it as the production of somatic mutations. We 

 have explained (see the quotation from E. Sutton Gersh) that the 

 difference is not one of principle but one of grade of function involv- 

 ing a narrow but not strict threshold for an all-or-none effect. There 

 is no reason to separate the complete or incomplete effect except 

 when discussing heterochromatin. 



For these reasons we cannot accept Lewis' (1950) statement (in 

 his review, which carefully avoids a discussion or even mention of 

 our point of view, as well as of any of the facts we found; as was 

 also pointed out implicitly by B. Glass in a critical review) that we 

 are dealing here with two completely different phenomena and that 

 the typical position effect (his S effect) is rare, while the motthng 

 effect (his V effect) is frequent and alone important. The variegation 



