GENE CONTROLLED PROCESSES 



189 



reared at the same temperature, we can measure the effect of the Bar 

 gene at that temperature. But if we simply have two flies, one Bar 

 reared at 25° C. and the other not-Bar reared at 15° C, we cannot 

 directly compare either the effect of the Bar gene as against not-Bar, or 

 the effect of the different temperatures, because the change from 25° C. 

 to 15'' C. may have different effects on Bar and not-Bar.^ 



For instance, one can imagine there being two genes A and B which 

 had exaaly the same phenotypic expression at 15° C. while at 25° C. the 

 expression of A remained the same while B gave a phenotype different, 

 say, by 10 units. No comparison between two of these forms could 



Fig. 89. Diagram of the 

 Dependence of Expres- 

 sivity on Environment. — 



Two (imaginary) genotypes 

 produce the same pheno- 

 type at 15^ C, but different 

 ones at higher tempera- 

 tures. 



TEMPERATURE 



reveal the true state of affairs; if we compared A at i^"" with B at 25° 

 we might think the whole difference was hereditary until we discovered 

 that A and B gave the same effect at 15°, or we might choose to suppose 

 the whole effect was environmental till we found that A and J5 at 25° 

 differed by exactly the same amount. Clearly there is no basis for either 

 opinion, and neither is correct. We can only find out the true state of 

 affairs by having two genetically identical stocks of A, one reared at 

 15° and one at 25°, and similarly two identical stocks of B reared at 

 these two temperatures. The special importance of this conclusion is 

 with reference to human genetics, since there we have to deal with 

 populations brought up under very different circumstances; but we 

 very rarely have genetically identical stocks which can be reared under 

 different conditions and thus provide a test of the effect of the environ- 

 ment as opposed to that of the hereditary factors. Ahnost the only 

 ^ Cf. Hogben 1933, Haldane 1936^. 



