NATURE OF THE GENETIC EFFECTS 401 



result to be expected when the amount of any reagent is increased which 

 has a hmited amount of material to work in and to exert its effect upon. 

 Moreover, since any of the numerous environmental conditions or genetic 

 agencies ("modifying genes") which can influence the activity or effec- 

 tiveness of the given gene, or gene-product, in producing its end result or 

 phenotype will have an effect like that of changing the dosage or activity 

 of the gene, it follows that such agencies, when acting on developing 

 individuals that have a low activity of the given gene, corresponding to 

 the left-hand, rapidly ascending region of the curve, will by sliding the 

 effectiveness backward or forward also slide the character up or down 

 correspondingly. Thereby its high variability at such levels is accounted 

 for. Conversely, in the right-hand, more nearly level, region of the 

 curve, the same influences, even though acting as strongly as before on 



^. NORMAL 

 Q. PHENOTYPE 



(ij 



X 



a 





I DOSE OF 2 DOSES OF 



NORMAL GENE NORMAL GENE 



GENE DOSAGE OR ACTIVITY 

 Fig. 7-6. Relation between gene dosage or activity and phenotypic effect in the case 

 of hypomorphic mutants. 



the biochemical processes concerned, will succeed in causing little or no 

 perceptible vertical deviation, representing alterations of the phenotype. 



Undoubtedly the normal genes were established through a long process 

 of natural selection of appropriate hypermorphs, in consequence of the 

 very fact that, because of the mechanism just discussed, such intense 

 activity gave the development of any given character that greater 

 stability which was advantageous to the organism and the species. That 

 the character, as normally developed, is at an optimum level in terms of 

 advantage for the organism, is readily demonstrated by tests showing the 

 lower ability of the mutant types to perform the functions in question. 

 The same conclusion is also supported by other cogent evidence, derived 

 from a phenomenon called "dosage compensation" (Muller, 1932b, 

 1950a), which is, however, too intricate to be explained here. In view 

 of the character being at its optimum level, it is also advantageous to 

 have its development proceed as reliably as possible for attaining pre- 

 cisely this most advantageous degree of expression in all individuals. 

 This situation, then, inevitably led to the selection of genes of sufficient 

 activity to operate in the near-saturation region of the gene activity- 

 character curve, for thereby the required stability would be attained as 

 nearly as possible. 



The actual phenotypic level at which the curve in question stands when 



