FERTILITY AND STERILITY IN DROSOPHILA 



TABLE 11 

 Sterility of the truncate stock as it appeared in the Fa generation 



147 



last two generations of this family. Fourteen pairs were chosen 

 from the F48 generation, two of which were sterile. From this 

 generation pairs were made up as shown in table 11. 



It should be recalled that the truncate stock was bred at the 

 same time and under the same conditions as the inbred stock, and 

 while the former developed into a fertile stock the latter developed 

 into a sterile female stock. 



Attention is called here to the fact that the individuals in table 

 2 are the grandchildren from a single pair of the F4 generation. 

 Sterility appears in the different famihes and affects about 50 

 per cent of the females. Family 27 is an exception in that 11 

 pairs show no sterility although descended from the same grand- 

 parents. This small amount of evidence is in accord with the 

 assumption that some individuals come through without the defect 

 in their germ-plasm, and shows how selection has brought about 

 its results. That one of the grandchildren in this combination 

 had some factor which prevented the appearance in her of the 

 defect is shown by the fact that the defect reappears among her 

 daughters (table 3, no. 134). These numbers are too small to base 

 a safe conclusion upon; but attention is called to them here, since 

 the asumption is borne out in the experiments which follow. 



BEHAVIOR OF STERILITY IN HEREDITY 



In a study of sterihty one meets at the outset a peculiar diffi- 

 culty in that he cannot breed the animals that show the very defect 

 which he wishes to study. Again there is no way by which sterile 

 animals can be recognized except by pairing them with different 

 mates which involves an additional amount of labor. Moreover 



