LINKAGE VARIATION IN DROSOPHILA 21 



couple, SO that positive interference will result. Let us now 

 consider two arbitrary points, M and N, so chosen that the dis- 

 tance between them is equal to the length of the average inter- 

 node. Whenever a node chances to occur near M, then the 

 next node will most often occur near N. That is, the chances 

 of a crossover at either point are very greatly increased by the 

 occurrence of one at the other point. In this case, instead of 

 getting less than the expected percentage of double crossovers, 

 we would expect, on the internode hypothesis, to get more 

 than the expected percentage. Interference for two such points 

 may be termed 'negative.' 



The second broods of the black purple curved cross give the 

 following data: 



B Pr Cv B I Pr Cv B Pr | Cv B ^| Pr | Cy 



2.215 96 366 16 



Percentage 82.24 3.57 13.6 .594 



From this we find that the interference is zero (percentage 

 of coincidence 100). There has been an 11 per cent rise in inter- 

 ference concomitant with the decrease in crossing-over. This 

 would suggest that the decrease in linkage here studied has 

 been due to an increase in the length of the average internode 

 rather than to a decrease in the percentage of chiasmas per node. 

 Unfortunately, the number of double crossovers obtained is 

 not great enough to establish this change in interference as 

 signif cant rather than due to chance fluctuation — i.e., the vari- 

 ation is within the limits of probable error. 



