LINKAGE VARIATION IN DROSOPHILA 19 



a convenient index, the converse of interference namely coinci- 

 dence, is calculated as the percentage which the observed double 

 crossover class is of the expected value. 



If, as described, interference is a function of the chromosome 

 twist, then from observing how the change in linkage here re- 

 ported affects interference, we can deduce the method by which 

 the linkage has been altered — whether by a decrease in the 

 percentage of breaks per node or by a decrease in the number 

 of nodes, that is, by a looser twist. 



If the twist remains normal and the decrease is due to a de- 

 crease from the normal percentage of breaks per node, then 

 each linkage value will be reduced proportionally. Whenever 

 a crossover does occur it occurs in the same position in which 

 it would normally have occurred, so that within any given 

 section of chromosome as great a percentage of crossovers would 

 be doubles as- in the normal condition. The effect is to make 

 the new condition a replica of the old, except that every cross- 

 over value is reduced in a common ratio. In this sort of change 

 in the mechanism, the interference would remain unaltered, for 

 in the ratio of the expected percentage of double crossovers to 

 the observed percentage, both terms decrease proportionally, 

 so that the value is unchanged.* 



If, however, the average looseness of the twist is increased, a 

 totally different result will be produced. Here the nodes become 

 actually further apart, so that whenever crossovers occur they 

 are no longer in the same average position as formerly but are 

 wider spaced. It then requires a longer section of the chromo- 

 some in order that double crossing-over be possible. This 

 means that for any definite section the number of double cross- 

 overs becomes less, that is, interference rises. 



On this second hypothesis, then, the closer spaced in relation 

 to the length of the internode the members of the pairs A-B and 

 C-D are, the higher is the interference. The new condition 

 should show the same interference as would be shown under 

 the old condition of a shorter internode by gens correspondingly 

 more closely spaced. By considering the amount of apparent 

 displacement (the decrease in the percentage of crossing-over) 



