THE MUTATED GENE 103 



as a simple quantitative deviation from Wild type toward insuffi- 

 cient production of something needed for perfect development. 

 Muller (1932a) has called this type of mutant gene hypomorphic. 

 It is the most frequent type of recessive mutation. We might 

 distinguish two possibilities of such actions. In cases of incom- 

 plete dominance, the degree of intermediacy might be shifted 

 experimentally. Or in cases of complete dominance, this might 

 be broken. A priori, the former type of effect would be expected 

 to be of general occurrence, being, after all, nothing but a special 

 case of the general type of modification. It is, however, to be 

 expected that the second type of shift is more difficult to accom- 

 plish, because complete dominance requires certain threshold 

 conditions (see page 117), with the result that even a considerable 

 shift does not carry the type below the threshold. 



Examples for the first group may again be taken from the 

 much studied case of vestigial wings in Drosophila. We recall 

 that the vg-w'mg may be changed by temperature action into the 

 phenotypes of all the other alleles. M. H. Harnly and M. L. 

 Harnly (1936) discovered a new allele pennant which has normal 

 or slightly notched wings and reacts strongly to temperature 

 changes in different ways. 1 The heterozygotes with v g may show 

 very different types according to the temperature at which they 

 are raised, fluctuating from vo-strap type at 26 to almost normal 

 at 32°. Here, then, temperature shifts the heterozygote exactly 

 as it shifts the homozygous vg-wing. The explanation in terms 

 of rates is obvious. It must be emphasized that the vg/ ' + - 

 heterozygote is usually Wild type in all temperatures (see 

 page 117). 



At this point we have to mention a phenomenon that resembles 

 a shift of dominance and has actually been confused with it 

 (see also preceding footnote concerning the same error by 

 Harnly). Hersh and Ward (1932), among others, compared 

 wing length and area in homozygous -f - flies and the hetero- 



1 M. H. and M. L. Harnly base their results upon measurements of wing 

 areas. This leads to erroneous results, the details of which will therefore 

 not be reported. His data show that the pennant wings are affected by 

 temperature considerably in general size, i.e., in cell growth after differentia- 

 tion. The measurements then lump together this influence upon secondary 

 growth with the effect upon the scalloping of the wing, which is a different 

 process. Goldschmidt's results in this respect could not have been known 

 to Harnly. 



