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 
