162 Nature of the Genetic Material 



efiFects or apparent point mutants were well-defined morphological 

 structures. This can be true only to a certain extent, namely, as far as 

 the Hmits of the segments can be made visible. However, there are 

 two phenomena which require a less morphological definition of the 

 segments. The first is the overlapping of neighboring segments, as 

 described above. If a break between bands a and b and between b 

 and c may produce the phenotypes of the segments to the right of c 

 as well as to the left of b (see fig. 11 for yellow and scute), it cannot 

 be the morphological segment which counts, but a field-Hke function 

 of the segment which under certain conditions — probably the residual 

 heredity and (or) features of the heterochromatin — reaches from its 

 center in the segment to different distances. The understanding of the 

 meaning of this requires the reaHzation that there is no gene or locus 

 within the segment, but only a definite polarized order on a molecular 

 level which functions or is disturbed in its function (mutation and 

 position effect) by a change in the structural sequence. If this function 

 has the character of a field, it may change in range under proper con- 

 ditions. A similar interpretation is hardly avoidable in the case of 

 the position effect of heterochromatic breaks which act upon a 

 series of sections to a considerable distance. The description of these 

 facts in terms of genes, namely, an action of adjacent heterochroma- 

 tin upon genes far away, provides no insight and is devoid of mean- 

 ing. Obviously, the phenomenon belongs in the same category as 

 the overlapping effect. Described in the same terms, it would mean 

 that the change in order, substituting heterochromatin for a more 

 or less distant euchromatic sector, stretches the fields of many ad- 

 jacent sections way down to the break. We could call this an extreme 

 type of overlapping. Such an effect is not so unexpected if we re- 

 member that heterochromatic changes (e.g., extra Y-chromosomes ) 

 can have a considerable influence upon many genetic actions (see I 2 

 C d ee). Distance actions within and between chromosomes are be- 

 coming better known and will in time contribute to a more definite 

 theory of all these effects, especially when studied without reference 

 to the classic gene. We mention only one: the rather mysterious 

 effect of inversions in one chromosome upon crossing over in an- 

 other (see Schultz and Redfield, 1951), an effect which is aheady 

 being used as a standard experimental method ( Lewis, 1952 ) . 



The time has not yet come to endow these discussions of the 

 chromosomal sections and the field-like type of their action with 

 a more definite biochemical meaning. But it should at least be men- 

 tioned that ideas have been proposed by others, quite apart from 



