Balance and Modification 449 



sexual only when strong F and weak M are very different. Otherwise 

 two weak M's will be needed to produce intersexual males which will 

 occur only in F2 or RF2. Thus we may say that in this sense females 

 become intersexual more easily. Of course, this, derivation is not a 

 theory but a description of an extensive set of experimental facts 

 involving all possible combinations. 



When in such combinations the males of the formula Fst/MstMw 

 are normal, and those Fgt/MwMw just a little intersexual, the proper 

 setting for the action of the modifiers is given, which may act in either 

 direction. Actually, two pairs of such dominant autosomal modifiers 

 were found, both acting in favor of M; as a result, only the homo- 

 zygous recessive segregants for one or both of these modifiers showed 

 low male intersexuality (details in Goldschmidt, 1923a). In the 

 triploid intersexes of Drosophila it was shown by Bridges ( 1925 ) and 

 Dobzhansky (1930) that modifiers for the degree of intersexuality 

 can be selected, though the case is not as clear-cut as in Lymantria, 

 where individual Mendelizing loci were demonstrated. Pipkin (1942) 

 found also in Drosophila dominant intersex modifiers in the X-chro- 

 mosomes of wild-type stocks of different geographic provenience. 

 The latter fact strongly suggests the parallelism with Lijmantria, as 

 does also the repleta case to be discussed later. We meet frequently 

 with the statement that these modifiers are autosomal sex genes, an 

 interpretation which has led to much confusion and the assumption 

 of multiple sex factors located all over the chromosomes. This would, 

 indeed, be a strange mechanism for accomplishing a simple genetic 

 alternative in the absence of complete linkage of all these autosomal 

 loci. The production of the two sexes in equal numbers would be a 

 very rare and improbable event, and if autosomal multiple sex deter- 

 miners for both sexes are assumed, as has been done, only a miracle 

 could lead to the regular production of the two sexes. From the point 

 of view of genie action, such theories are even worse than from the 

 purely statistical aspect. The serious error in this confusion of the 

 sex-determining mechanism and the possibility of modifying effects 

 upon the action of the primary F/M balance can be made most 

 obvious by comparison with facts outside the realm of sex deter- 

 mination. 



In a former section I mentioned dominance modification. Among 

 innumerable examples (all the inhibitors, suppressors, enhancers in 

 genetic literature) are the accurately studied dominance modifiers for 

 vestigial wings in Drosophila (see III 5 C b). One of them, which 

 shifts all combinations and compounds in the direction of more wing 



