CROSSINGOVER IN DROSOPHILA 



165 



crease in the percentage of crossingover in the second chromosome. 

 Sturtevant has discovered a definite Mendelian factor which 

 entirely prevents crossingover in the second chromosome be- 

 tween the factors star and black, and considerably reduces it 

 between black and purple. Between purple and curved its 

 presence results in a rather increased percentage over the nor- 

 mal control. This 'little crossover' factor is known as C II 1. 

 It was possible that increased temperature might cause some 

 crossingover in the star-black region. The test was made with 

 characters which involved the whole length of the chromosome 

 so far worked out. In addition to C II 1, star, black, curved and 

 speck were involved in the experiment. A glance at the chromo- 

 some map (fig. 1) will show the relative positions of these factors 

 constructed on the basis of numerous experiments with the 

 ordinary stock. The Pi parents in the test were star-black- 

 males mated to females having C II 1, curved and speck, kindly 

 supplied by Dr. Sturtevant. The star Fi females were back- 

 crossed to males of the multiple stock black-curved-speck. 

 The results were as follows: 



TABLE 9 

 S' b 



cm 



c sp 



FEMALES HATCHED AT 



Control, 22°C 



Second broods after 10 days at 22° 



Heat, 31.5°C 



Second broods after 10 days at 22° 



c — sp 

 per cent 



35.3 

 32.3 

 32.0 

 29.0 



*No crossovers between S 1 and b in any case. 



The summary shows conclusively that far from causing any 

 crossingover in the star-back region the heat treatment appar- 

 ently accentuates the effect of the 'little crossover' factor. The 

 diminution in the percentage of black to curved in the heat 

 series is not great enough to be very significant but at least it 

 makes it certain that heat does not cause the expected increase, 

 and is not able to overcome the effect of C II 1. The column 



