THE MUTATED GENE 71 



single mutation but seems to be caused by a major differential 

 factor and some additional minor factors. By selection and 

 inbreeding, different lines were isolated, characterized by different 

 percentages of penetrance of the character. In this case, it was 

 also proved that the normal individuals had the same genetic 

 constitution as the four-toed ones. Some of the facts pointed 

 in the direction of external influences upon the percentage inci- 

 dence of the type, but mammals are unsatisfactory material for 

 such experimentation. Wright realized also, just as Goldschmidt 

 had in the cases reported before, that the problem comes down 

 to the question of a physiological threshold, which is reached or 

 not reached under certain combinations of genetic as well as 

 environmental conditions. 



A systematic inquiry into this problem has been made by 

 Timofeeff (1925, 1929a, 6, 1934) who worked with Drosophila 

 funcbris, a species in which most of the mutants do not show a 

 100 per cent effect but a lower percentage of penetrance under 

 standard conditions. The mutant in question is venae transversae 

 incompletae (vti) of D. funebris, exhibiting an interruption or 

 absence of the cross veins of the wing. Vti is an autosomal reces- 

 sive which requires in addition the presence of another gene for 

 visible manifestation. Furthermore, there are present other 

 modifying genes which influence the percentage of individuals 

 exhibiting the defect, i.e., the penetrance. By means of selection, 

 lines could be isolated which showed from 41 to 100 per cent 

 penetrance, respectively. (There could also be isolated lines 

 differing in the quantitative amount of the defect and in its 

 specific pattern and symmetry. These facts do not pertain to the 

 problems of this chapter.) With these selected lines, experiments 

 were performed which proved an influence of temperature action 

 upon the percentage manifestation, but only in some of the lines. 

 In general, the percentage of manifestation decreases with increase 

 in temperature. Thus 13°C. gives 97 per cent, and 28° gives 

 74 per cent if the temperature is applied during a sensitive 

 period in the first half of larval life. But there is another sensi- 

 tive period in pupal life where temperature acts reciprocally; a 

 rise in temperature produces a small increase in penetrance. 

 The explanation that Timofeeff gives for these facts falls in line 

 with all the general conclusions that we have presented in former 

 chapters. During the early sensitive period, the processes that 



