8 PHYSIOLOGICAL GENETICS 



method would produce the phenocopies of all known mutants 



(sec pages <) and 10). It ought to be added that in these experi- 

 ments the modified individual may exhibit simultaneously quite a 

 number of different traits; e.g., the wing might he simultaneously 

 short, spread, rolled, and even the two wings different. 



The following details are of importance. Within a rather 

 considerable range of variation, typical effects are dependent 

 upon four variables: 



1. Developmental stage at which the heat shock is applied. 



2. Time of exposure to heat (6 to 24 hr.). 



3. Intensity of heat (35 to 37°). 



4. Genetic condition of the material. 



It might be said that the number of phenocopies (up to 100 per 

 cent) and the degree of phenocopic change (up to the highest 

 member of a given series) are roughly proportional to the product 

 of the time and the temperature of the exposure, i.e., to the 

 intensity of the shock. The specific type of phenocopy produced 

 is dependent upon the intensity of the shock, the time of its 

 application, and the genetic line used for the experiment. It 

 would be surprising, of course, if so rough a method were to 

 produce a 100 per cent typical result for all different combinations. 

 But it is a fact that, at least for some of the phenocopies, a formula 

 can* be given which always produces them within a certain range 

 of variation. Table 1 contains some data as found in a defi- 

 nite series of experiments with the Oregon wild line. 



It is further very typical that certain phenocopies tend to 

 appear together with a definite treatment. Thus it is possible to 

 produce quite regularly the combination of spread wings and 

 long (angora) bristles or of curved wings and short stubbly 

 bristles. 



The influence of the race is a rather considerable one. Some 

 wild or mutant races respond more easily to the stimulus than 

 others. Some are more apt to produce phenocopies of the 

 bristles; others, with the same method, give more wing modifica- 

 tions. And some of the phenocopies were produced exclusively 

 or almost exclusively within a definite line. We shall return later 

 to this point. 



A special interest attaches to the time element, the sensitive 

 or critical period at which the phenocopies may be produced. 

 This important point will be treated in a separate chapter. 



