68 LINKAGE 



crossing over represents precisely the distance between 

 the factors, for it may be that crossing over is more 

 likely to take place in one region of the chromosome 

 than in another. In that case the distances between 

 factors in this region calculated from the amount of 

 crossing over between them, would be relatively 

 greater than the actual distance. It is supposed, 

 however, that at least the order of the factors in the 

 diagram represents their real order. Sturtevant 

 has found definite factors which alter the amount 

 of crossing over in the chromosomes, and these factors 

 actually do affect the amount of crossing over differ- 

 ently in the different regions. A map of the chromo- 

 somes based upon the per cent, of crossing over when 

 these factors are present would show different rela- 

 tive distances between the loci than those calculated 

 from the normal linkage values. It is to be noted, 

 however, that even in these diagrams, the order of 

 the factors remains unchanged. One of the factors 

 lies in the second chromosome and lowers the amount 

 of crossing over in certain regions of this chromosome ; 

 the other lies in the third and apparently affects 

 only this chromosome, and chiefly the end of this 

 chromosome in which it itself is located. Bridges has 

 found that the percentage of crossing over in the sec- 

 ond chromosome is also lowered with increase in the 

 age of the female, and it may be that other influences 

 as well may affect the amount of crossing over. 

 This variation in crossing over is in no way preju- 

 dicial to the conception of crossing over above out- 

 lined. Variation in the amount of crossing over has 



