THE MUTATED GENE 109 



work. He tested (from the standpoint of his phylogenetic 

 theory, see page 121) the dominance of the dominant mutations 

 in fowl. He found that after repeated backcrossing to the wild 

 jungle fowl such genes were not dominant any more, probably 

 because by this time some dominigenes, necessary for dominance, 

 had been removed. 



In some cases, the existence of dominigenes was proved by 

 selection. Timofeeff (1925, 1934) selected for modifiers of two 

 recessive abnormalities of the venation in Drosophila funebris 

 and succeeded in establishing lines with incomplete recessiveness. 

 A similar result was obtained by Stanley (1935) for the recessive 

 vestigial wing. Finally, some cases ought to be mentioned that 

 are more complicated because chromosome abnormalities are 

 involved. Mohr (1929a) showed that the effect of the dominant 

 deficiency Gull in Drosophila is lessened by the presence of the 

 gene dachsous. Of course, a deficiency cannot strictly be com- 

 pared with a heterozygote; this point will be discussed in another 

 chapter. A similar observation is to be made, when it is reported 

 that the addition of a duplicated fragment of a Wild-type 

 chromosome in Drosophila to the recessive line does not result 

 in a completely wild type (Dobzhansky and Sturtevant, 1932). 

 Many possibilities are given to explain such cases within our 

 general model. A very different point of view, the so-called 

 position effect, will be discussed later in connection with the same 

 facts. 



The most thorough analysis has been made thus far by Gold- 

 schmidt (1935-1937) for the recessive character vestigial wings 

 in Drosophila. Normal wings are completely dominant over 

 vestigial. It was possible to isolate three dominigenes which 

 shift the dominance toward the vg-type if all three are present. 

 These are one sex-linked recessive allele of the cut gene and two 

 autosomal dominants in the second and third chromosomes. 

 If only the autosomal dominants are present in the vg/ + hetero- 

 zygote, a small dominance effect is produced; i.e., about 1.1 per 

 cent of the individuals are nicked. The cut allele in homozygous 

 condition affects similarly about 0.5 per cent individuals. If, 

 however, all three dominigenes are present, all vo/+ heterozygotes 

 are scalloped. The autosomal dominigenes in this case have an 

 additive effect : for the degree of scalloping, namely nick to notched, 

 (see page 57) increases from AB/ab to AB/AB. A gene was also 



