FERTILITY AND STERILITY IN DROSOPHILA 159 



that it was also present in the truncate stock at this time is shown 

 not only from the controls but also from the crosses themselves. 

 It looks at first sight as if the sterility reappeared in a higher de- 

 gree in the reciprocal than it did in the cross, since it occurs in 32 

 cases out of 99 in the reciprocal while in the cross the sterility 

 occurs in 14 pairs of the 84 pairs tested. It is to be noted, how- 

 ever, that the pairs as made up are not equally distributed, for, 

 number 29 of the cross has 33 representatives none of which are 

 sterile. If correction is made for this we have 14 sterile pairs in 51 

 or 28 per cent which corresponds fairly well with the 33 per cent 

 of sterility as it occurred in the reciprocal cross. The fact that 

 33 pairs of number 29 showed no sterility is significant as has 

 already been pointed out; for, it shows that some families do not 

 show the defect, while in closely related families the defect may be 

 present to a high degree. 



In table 18 I have compiled the results of all the experiments 

 that deal with the transmission of sterility. A census of the 

 whole situation shows that of 417 pairs set aside as controls 113 

 pairs were sterile. Twenty-seven per cent of the controls were 

 sterile. There were 84 sterile females and 14 sterile males. Of 

 the 149 crosses made between the sterile and fertile strains 35 

 pairs, or 24 per cent, were sterile. If the sterility behaves any- 

 thing like a recessive character the expectation is that sterility 

 will not appear in the Fi generation. A total of 832 pairs tested 

 in the Fi generation verifies the expectation as only 19, or 2.3 per 

 cent, are sterile. These seem to be chance occurrences as both 

 sexes are equally affected — 5 females and 6 males. If one could 

 find an absolutely fertile race against which to test the sterility, 

 it is altogether probable that no sterility at all would appear in 

 the Fi generation. The sterility as it appeared here in the Fi 

 generation apparently gives a fair measure of the degree to which 

 the sterility entered into the experiment from the strain against 

 which the sterility of the sterile strain was tested. 



In the F2 generation we find that the sterility as it affects the 

 female reappears. Of the 2644 pairs tested, 407 pairs, or 16 per 

 cent, proved sterile. There were 237 sterile females and 60 

 sterile males. The rather large number of males given here in- 



