242 MULTIPLE FACTORS 



low as and rarely higher than 20 per cent.; in 

 later generations, however, the proportion and the 

 intensity of the extracted truncates may again be 

 raised, by selection, to about 90 per cent. That the 

 variation in truncation within the original highly 

 selected stock, as well as within the extracted stock, 

 is not merely a somatic effect due to uncontrollable 

 environmental influences was shown by the fact 

 that the variations in both lines were to a large 

 extent heritable: selection of normals resulted in a 

 much lower percentage of truncate offspring than 

 were thrown by their truncate brothers and sisters, 

 and selection of intermediates gave, on the average, 

 intermediate results. The fact that such genetic 

 differences are still constantly occurring in the 

 original stock, in spite of the long-continued se- 

 lection, seemed to indicate that here at least there 

 was a case of instability of factors or contamination 

 of allelomorphs. 



An analysis of the factorial composition of the 

 truncate flies was then made by crossing them to 

 flies containing in each of their chromosomes other 

 mutant factors whose hereditary behavior was 

 known. In the second generation of the cross 

 (back-cross) these other factors served as identifying 

 marks which disclosed just which chromosomes of 

 the Pi truncate fly each ¥. individual had or had 

 not received. By observing the amount of trun- 

 cation which accompanied each ascertained combi- 

 nation of chromosomes it could thus be determined 

 just what role each of the chromosomes played in 



