SEX-LINKED FACTORS IN DROSOPHILA 47 



TABLE 1 



Long vermilion 9 — ^MpX MpX 



Rudimentary red cf' — mPX 



Fi MpX mPX— long red 9 



MpX — long vermilion c^ 



Gametes Pi 



Eggs —MPX mPX MpX mpX 



Sperm— MpX 



MPX MpX \ , , n .ri 



mPXMpX/-^^"^^"^ ^-^^^ 



MpX MpX t , ... n 11- 



„ ,, ^, ) — long vermiuon 9 — -ll/ ■ 

 mpX MpX J 



MPX —long red a"— 105 



mPX — rudimentary red cT — 33 



MpX — long vermilion cf — ^316 



mpX — rudimentary vermilion cf — 4 



and is being investigated by Morgan. The point of interest at 

 present is the hnkage. In the F2 generation the original com- 

 binations, red rudimentary and vermihon long, are much more 

 frequent in the males (allowing for the low viability of rudiment- 

 ary) than are the two new or cross-over combinations, red long 

 and vermilion rudimentary. It is obvious from the analysis 

 that no evidenge of association can be found in the females, 

 since the M present in all female-producing sperm masks 

 m when it occurs. But the ratio of cross-overs in the gametes is 

 given without complication by the F2 males, since the male- 

 producing sperm of the Fi male bore no sex-linked genes. There 

 are in this case 349 males in the non-cross-over classes and 109 

 in the cross-overs. The method which has seemed most satis- 

 factory^ for expressing the relative position of factors, on the theory 

 proposed in the beginning of this paper, is as follows. The unit 

 of 'distance' is taken as a portion of the chromosome of such 

 length that, on the average, one cross-over will occur in it out 

 of every 100 gametes formed. That is, percent of cross-overs 

 is used as an index of distance. In the case of P and JM there 

 occurred 109 cross-overs in 405 gametes, a ratio of 26.9 in 100; 

 26.9, the per cent of cross-overs, is considered as the 'distance' 

 between P and M. 



