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 d*. 



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. 



