36 SEX-LINKED INHERITANCE IN DROSOPHILA. 



Table 8.— Pi a-ild 9 9 X yellow sable cTcf • fi wild-type 9 9 X 



F\ wild-type cf cT . 



Some of the Fi females were back-crossed to yellow sable males and 

 gave the data for table 9. 



Table 9.— Pi wild-type 9 9 X yellow sable d'd'. B.C.Fi wild-type 9 X 



yellow sable cf cf . 



In these tables the last column (to the right) shows for each culture 

 the amount of crossing-over between yellow and sable. These values 

 are found by dividing the number of cross-overs by the total number of 

 individuals which might show crossing-over, that is, males only or both 

 males and females, as the case may be. Free assortment would give 

 50 per cent of cros.s-overs and absolute linkage o per cent of cross-overs. 

 Except where the percentage of crossing-over is very small these values 

 are expressed to the nearest unit, since the experimental error might 

 make a closer calculation misleading. 



The combined data of tables 8 and 9 give 686 cross-overs in a total of 

 T,6oo individuals in which crossing-over might occur. The females of 

 table 8 are all of one class (wild type) and are useless for this calculation 

 except as a check upon viability. The cross-over value of 43 per cent 

 shows that crossing-over is very free. We interpret this to mean that 

 sable is far from yellow in the chromosome. Since yellow is at one end 

 of the known series, sable would then occupy a locus somewhere near 

 the opposite end. This can be checked up by finding its linkage rela- 

 tions to the other sex-linked factors. 



