116 PHYSIOLOGICAL GENETICS 



If, then, dominance is considered for all effects of one allele, 

 it acts simultaneously as a dominant on one character and as a 



recessive upon anot her. An example from animals is the truncate 

 series in Drosophila with consistent but not parallel actions upon 

 wings and thoracic hair. In such cases, a special phenomenon 

 may occur, if some of the alleles act only upon one of the traits. 

 In the truncate series, the high allele T v0 does not affect the wings 

 but the hair. The intermediate allele T° affects the wings but 

 not the hair. In the wing series, T vo is dominant over T°; in 

 the hair series, T° is dominant over T v0 . The compound of both 

 then produces a normal phenotype. There are finally the cases 

 in which the action of multiple allelomorphs does not appear in 

 a quantitative series, e.g., the spineless and aristapedia alleles 

 of Drosophila, the first affecting the bristles; the second, a 

 homoeosis of the antenna. Here rather independent behavior 

 of dominance in regard to the different traits occurs. A special 

 type may be found if, as in the scute series of Drosophila, 

 the alleles affect patterns of the same structure; each allele 

 prevents the formation of definite bristles. In this case, the 

 presence of the individual bristle in the pattern is always domi- 

 nant over its absence, from which it follows that the compounds 

 are always, more like the Wild type than the effect of both ingredi- 

 ent genes would be. And if two alleles affect reciprocal parts 

 of the bristle pattern, their compound must be Wild type. But 

 such cases, though important in other respects, do not furnish 

 special information for the problem of dominance. 



We have now to find out whether such facts regarding dominance 

 in multiple allelomorphic series furnish information on the mean- 

 ing of dominance and therefore throw light upon the problem 

 of the action of the gene. To do this we shall discuss first such 

 cases as, according to facts already reported, permit one to draw 

 definite conclusions. The best case in this respect is made out for 

 the vestigial series. The dominance relations in this case are 

 of the type as reported under 4 in the foregoing enumeration. 

 Wild type is dominant over all alleles except the lowest member 

 of the series, No-wing, which, in addition, is lethal in a homozy- 

 gous condition. All the other compounds are intermediate and 

 fit into their proper places between the homozygous alleles, 

 as may be seen from Table 7 (page 80) and also from Fig. 24. 



