THE NATURE OF THE GENE 



371 



Sturtevant and Schultz^ showed that the effects of scutes are increased 

 when they are combined with the iiird chromosome gene Hairless, 

 which acts as a sensitizer by lowering the threshold for the bristle- 

 suppressing action of scute. In such circumstances, a scute allelomorph 

 is found to affect many more bristles than it normally does, and these 

 may be scattered more or less at random over the sub-gene map. 

 Similarly, Child^ showed that the set of bristles affected by a scute 

 allelomorph depends on the temperature during larval life. In both 



10-^^^ 



mr a'dcp'dc iv so. dv vt pnp p3 anp por a'pi ppa dor mr oc a pv asc p5c st h 



Fig. 150. Effects of Scute-1 at Different Temperatures. — In the bottom row, 

 the bristles are arranged in the same order as in Fig. 1^9. The three curves above 

 shov/ the effect of scute-1 in removing bristles in flies reared at 14-°, 22°, and 28° C. 

 The effect on males, where it differs from that on females, is indicated by the 

 dotted lines. Note that at the high and low temperatures, scute-1 may affect 

 bristles which lie outside its normal region according to Fig. 149. 



(From Child.) 



these cases, the fundamental postulate that a scute allelomorph affects 

 only a particular set of bristles is found to break down. 



The remarkable data about the scutes still await a full solution. It is 

 now known that scute-Uke effects can be the result of transpositions of 

 genes in this region of the chromosome (position effects). It seems 

 likely, however, that the explanation of the different patterns of effect 

 of the different allelomorphs will have to be sought in a developmental 

 theory rather than in the geometrical structure of the gene or the 

 chromosome. Purely formal attempts to provide such developmental 

 theories have been made by Goldschmidt^ and Sturtevant and Schultz 

 in terms of the diffusion of a "bristle forming substance." There is 

 some difficulty, however, in formulating a theory which fits the facts, 

 and still no experimental evidence that such a substance exists. 



^ Sturtevant and Schultz 193 1. ^ Child 1935. ^ Goldschmidt 1931. 



