170 THE CHROMOSOMES 



Special attention must be called here to two facts 

 which have been repeatedly pointed out above, and 

 which have long been commonplaces in the literature 

 on linkage in Drosophila. First, owing to the ex- 

 istence of multiple crossing over, it is not true that 

 the distances between the factors in the straight 

 linear map have the same numerical values as the 

 observed percentage of recombination. Secondly, 

 owing to the reduction in the amount of multiple 

 crossing over caused by interference, it is not true 

 that the relation between map distance and recombi- 

 nation is that which would result if crossings over 

 were independent of each other ("Trow's formula" 

 assumed a relationship of linkages of this sort, but 

 it had already been disproved in Drosophila). 

 Haldane has recently restated the evidence against 

 these misconceptions in slightly different termi- 

 nology, under the erroneous impression that the 

 first of the misconceptions is held by Drosophila 

 workers. In substitution for these two views he 

 proposes an empirical formula to express the relation 

 supposedly existing between distance and separation 

 frequency. Such an attempt to find a general 

 formula is futile, for it has been proved that the 

 relationship between map-distance and observed 

 percentage of crossing over is different not only in 

 different chromosomes, but even very strikingly in 

 different regions of the same chromosome. The only 

 satisfactory representation of such relationships is 

 one that gives for each chromosome and for each 

 region of that chromosome the observed relation 



