196 



AN INTRODUCTION TO MODERN GENETICS 



Stream to halt at a stage which is attained and surpassed in normal 

 development. It may be that the stream flows more slowly in Sy moths, 

 and therefore does not spread so far in the time available, or it may be 

 that there are forces opposing the stream and these are intensified by 

 Sy. A recessive gene syb has a similar effect. 



The final spread of the determination stream can also be affected by 

 high temperatures (45° C.) applied for three-quarters of an hour during 

 the pupal period. These temperature shocks do not fix the stream at the 

 stage it has reached at the moment of shock, as do the cauterizations. 



ABC D E 



Fig. 92. Genetic and Environmental Control of Wing Patterns in Ephestia. 



— The drawings show wings from pupae which were heated to a moderate tem- 

 perature {A5^ C. for A5 minutes) at various stages of development. A and 6 are 

 modifications in moths with the constitution Sysy, whose normal appearance 

 is C; but C can also be obtained as a modification of sysy, whose normal appearance 

 is D or £. 



(From Kuhn.) 



but actually alter the final equilibrium which the stream attains after 

 diffusion. If the stimulus is applied during the first 36 hours of pupal 

 Hfe, the stream is reinforced and the mid-field broadened; a Sysy 

 genotype can be caused to develop like a normal sysy. Between 48 and 

 60 hours the effect is reversed, and the mid-field contracted. Later the 

 reactivity of the wing falls and the stimulus produces no great effect. 

 When genetic types are imitated by environmental agencies such as 

 temperature the results are known as phenocopies (p. 191). Unfor- 

 tunately, it is still quite obscure in the above case how the temperature 

 actually affects the determination process and we cannot yet draw any 

 conclusions about the mode of action of the genes. 



By the study of temperature eflfects it is possible to analyse compli- 

 cated wing patterns into the physiological elements of which they are 

 composed. In Vanessa urticae,^ for instance, there is a general tempera- 

 ^ Kohler and Feldotto 1935. 



