210 



THE SECOND-CHROMOSOME GROUP 



developed gaps in a total of 115 flies, or 30 per cent. If the gap Fi 

 mother had been homozygous for gap and the not-gap father heter- 

 ozygous, 50 per cent of gap offspring would have been expected. 

 Cultures M 37, II 41, and 397 each gave very nearly a 2 not-gap to 1 

 gap ratio. Their total was exactly 200 not-gap to 100 gap. 



No satisfactory conclusion has been drawn from these data, though on 

 the whole, gap seems to be a recessive character, and there is probably 

 present in the original black-arc stock some special modifier or rela- 

 tionship that makes gap appear in the Fi of the cross. There seems 

 also to be a sex-limited or sex-linked difference. 



The gap stock is still (April 1918) on hand and shows the same 

 condition of the character after 5 years of unselected culture. 



Table 56. — Pi, gap black arc 9 X black arc cf . 



CHROMOSOME AND LOCUS OF GAP. 



The evidence that gap is second-chromosome consists solely in the 

 persistence with which gap has accompanied black through certain 

 crosses. The fact that a cross-over between black and gap in getting 

 black arc stock retained gap with the black suggests that the locus of | 

 gap is to the left of arc and perhaps near curved. 



4 



