Genie Control of Development 359 



basis of different developmental systems, why different dosages may 

 fail to produce different phenotypes. We may also expect that some 

 developmental processes might be of such a kind as to produce, in 

 spite of the different developmental systems, different sexual pheno- 

 types, which is known to be true of a number of loci. This means that 

 in the latter case the loci in question act simply according to their 

 dosage. The explanation which offers itself at once is that in both 

 cases threshold phenomena are involved which lead to one or the 

 other type of behavior. The entire problem becomes the same as that 

 of inheritance of sex-controlled characters, for example, when a defi- 

 nite phenotype, sometimes controlled by a single mutant locus, shows 

 up in only one sex (of course, speaking of cases without hormonic 

 control). The sex-determining mechanism, acting through the control 

 of the balance of action of female and male sex determiners, is re- 

 sponsible not only for the course of sexual differentiation but also for 

 all the concomitant differences in metabolism and rate of differentiat- 

 ing processes. These differences are known to occur when other 

 mutant loci produce their special effect. For example: in many moths, 

 males develop faster than females (details in Goldschmidt, 1933a, 

 for Lymantria dispar). Sex-linked mutant loci may therefore be ex- 

 pected to produce the same effect in simplex and duplex condition 

 when the sex-controlled changes in the developmental system of the 

 simplex sex run parallel to the dosage change of mutant action. If, for 

 example, the reaction produced by one portion of a mutant reaches 

 the threshold of action later than in the case of two doses, the effect 

 of both will be the same if the developmental system of the simplex 

 sex is such as to fit in with later action of the respective locus. If not, 

 the simplex effect will be different from the effect of two doses: lower 

 if the shift is in one direction; higher if it is in the other direction. 

 Such a scheme covers all known behaviors of sex-linked mutants in 

 one or two doses, without recourse to a special explanation (like 

 dosage compensation) for each variant. Thus the action of sex-linked 

 mutants in one or two doses does not lead — for our problem of genie 

 action as revealed in dosage experiments — to conclusions other than 

 those studied before for the Bar, bobbed, and cubitus interruptus 

 experiments. Recently, Cock (1953) has studied the same problem in 

 the barred breeds of fowl, used for autosexing, because the difference 

 in the barring of plumage m the two sexes, based upon a sex-linked 

 dominant, is already visible in the young chick. He shows that dosage 

 compensation (which would exclude autosexing) is not present and 

 that Muller's explanation does not work in this case. Obviously, the 



