LINKAGE VARIATION IN DROSOPHILA 17 



must suppose that normally the tightness of twist for any chro- 

 mosome varies, within limits, in the different maturing cells. 

 The length of the section between nodes — the internode — will 

 hence vary correspondingly, but around a modal length normal 

 for that chromosome. It is also probable that crossing-over 

 does not take place at every node but only in a certain per- 

 centage, specific for the chromosome. 



If the average length of the section between nodes, the inter- 

 node, remains unchanged (that is, if twisting becomes neither 

 looser nor tighter) such a decrease in the amount of crossing- 

 over between given gens can be explained as failure to break 

 and re-fuse, i.e., cross over, at as many of the nodal points as 

 normally. 



If, on the other hand, the average length of the internode 

 becomes greater (that is, if twisting becomes looser) such an 

 effect as described would be produced while the percentage of 

 breaks per node remains constant. A possible means of de- 

 termining which of these views obtains here is offered by a study 

 of the interference effects in the first and second broods. 



Interference stands in about the same relation to linkage as 

 linkage does to free Mendelian assortment. In hnkage there 

 is a hindrance to the independent assortment of two pairs (A, a 

 and B, h) of allelomorphic gens. Such a case in sweet peas is 

 that of round pollen versus long (pair A, a) and red flower 

 versus purple (pair B, b). In the phenomenon of interference 

 there is a hindrance to the independent linkage of the members 

 of two couples of linked genes, A-B and C-D. In any case of 

 free Mendehan assortment one expects as great a percentage 

 of A to be at the same time 6, as of a to be at the same time B; 

 that is, AB : Ab :: aB : ab (as, e.g., in 9 : 3 : 3 : 1 or 1 : 1 : 1: 1). 

 In cases of linkage this relation is altered by a deficiency of the 

 classes arising by crossing-over. Likewise in cases where two 

 couples A-B and C-D in the same linkage group (chromosome) 

 are crossed together, the number of individuals in the double 

 crossover class may be considerably smaller than expectation 

 according to the proportion above. 



THE JOtTRNAL OF EXPERIMENTAL ZOOLOGY, VOL. 19, NO. 1 



