356 Action of the Genetic Material 



be taken as an illustration how little biochemical genetics has helped 

 us so far in the explanation of genie action outside the small group of 

 production of metabolites. 



cc. Dosage in sex linkage 



The dosage problem comes up most naturally in the cases of sex 

 linkage in which the homogametic sex has two doses of a mutant locus, 

 while the heterogametic sex has only one dose. If simple dosage 

 relations are at play, as we had to assume when comparing homo- and 

 hemizygotes (n/n vs. n/0), the phenotype in the two sexes should 

 show this difference. Actually, the phenotype is frequently alike in the 

 two sexes in regard to a sex-linked mutant. Stern (1929) first con- 

 sidered this situation and came to the conclusion that it must be the 

 result of a selection of modifiers for an identical phenotype of the two 

 sexes, this being of adaptive value. Muller (1933, 1950b), who often 

 returned to this problem, speaks of dosage compensation. 



There can be no doubt that some mutant loci in the X-chromo- 

 some (e.g., of Drosophila) act alike in both sexes, but there can be no 

 doubt either that other loci show sexual differences. What is still more 

 important, multiple alleles of a locus with sexual differences of ex- 

 pression may be sometimes more extreme (toward the mutant side of 

 expression) in the homozygous sex; and, in other cases, more extreme 

 in the heterozygous sex; and, finally, some may be of equal expression 

 in both sexes. The best-known example is that of the multiple alleles 

 at the white locus in Drosophila. As we stated earlier {1 S C c ee ccc), 

 it has recently been shown that two pseudoallelic loci are involved 

 here, the white and the apricot locus. In all the alleles of apricot, the 

 simplex condition ( 5 ) is darker than the duplex ( 9 ); in aU alleles 

 of white, the males are either lighter than the females or they are 

 identical. In this connection, another fact should be emphasized. We 

 showed a long time ago (1921a) that melanism (dark wings) in the 

 nun moth is caused by a major sex-linked factor and two minor auto- 

 somal ones, all of which act toward increase of pigment according to 

 dosage. In this paper a plate is found with photos of such a dosage 

 series of females and males side by side, and it can easily be seen that 

 the sex-linked mutant ( and also the additional autosomal ones ) always 

 acts stronger in the males, all combinations of males being one grade 

 of darkness deeper than females of comparable constitution. Now in 

 this case the female is heterogametic, and thus one dose is less strong 

 than two doses, and the additive action of the homozygous autosomal 

 pigment factors does not change this situation. Hence no dosage 



