THE MUTATED GEM: 131 



efficiency of the gene in simplex condition. But there must 

 be some reason for the existence of these dosage compensators, 

 which cannot be expected to be present for eventual action upon 

 mutations. Muller therefore assumes that they have also a 

 similar action upon the Wild type, which thus is held high above 

 its minimum threshold effect, and that consequently these modi- 

 fiers have been evolved by selection after the manner of Fisher's 

 theory (but independently of heterozygosis). 



3. The interpretation introduced above to explain sex- 

 controlled effects also explains the facts being considered here. 

 This seems to me the most reasonable explanation. The sex- 

 determining mechanism, acting through the control of the 

 balance of the sex genes, determines not only the course of sexual 

 development but simultaneously all the concomitant differences 

 in metabolism and rate of differentiating processes. These 

 differences are known to occur at the time when many mutant 

 genes produce their phenotypic effect. For example, in many 

 moths the males develop faster than the females, but pupal 

 development takes longer in males (exact data in Goldschmidt, 

 1933a). Sex-linked genes will therefore produce the same effect 

 in a simplex condition as in duplex, when the sex-controlled 

 changes in the developmental system of the simplex sex run 

 parallel to the change in gene action from duplex to simplex. 

 If, for example, the reaction produced by one portion of a mutant 

 gene reaches the threshold of action later than in the case of two 

 portions, the effect of simplex and duplex will be the same, if 

 the whole system of the simplex sex is such as to fit in with a 

 later action of the respective gene. If this is not the case, 

 the simplex gene will have a different effect — a lower one if the 

 shift is in one, an even higher one if the shift is in the other, 

 direction. This explanation which covers sex-linked genes both 

 with or without rate effect, also with increased effect of simplex, 

 and which fits this phenomenon into the whole system of action 

 of genes, as revealed in physiological genetics, seems superior 

 to the phylogenetic explanations of dosage compensation. 



If this is the case, the difference or lack of difference in the 

 effect of one or two doses of sex-linked genes furnishes no direct 

 results for the problem of the action of genes in different quanti- 

 ties. The effect, whatever it is, is not produced under the condi- 

 tions of ceteris paribus but in two different physiological systems. 



