LINKAGE 



169 



other units) corresponding to the average cross-over percent- 

 age between the two, this process being repeated from gene 

 to gene until the whole chain is plotted. The "map" is thus 

 based on a summation of the distances (measured in cross- 

 over percentages) from gene to gene. But if we compare the 

 "map distances" between genes not adjacent to each other 

 in the chain with the observed cross-over percentages be- 

 tween the same genes, we find that the map distance is regu- 

 larly greater than the cross-over percentage, except for very- 

 short distances (5 or less) . Thus if three genes occur in the 



I 



i 



w 



JH 



Fig. 121a. B and C illustrate Morgan's idea of 

 the linear arrangement of the genes in the chromo- 

 somes. A and D show how the composition of 

 chromosomes is supposed to change as a result of 

 a crossover. Fig. lli. A pair of homologous 

 chromosomes a, before; fc, during; c, after a double 

 crossover. (After Morgan.) 



re- 



order A, B, C, it is usually found that AB + BC is greater 

 than AC. In other words, the cross-over percentage be- 

 tween A and B plus the cross-over percentage between B 

 and C is commonly greater than the cross-over percentage 

 between A and C, and the discrepancy increases with the 

 magnitude of the values involved. This fact has been 

 accounted for in two different ways. First, it may be sup- 

 posed that the arrangement of the genes is really not linear, 

 that B lies out of line with A and C, so that AC will be less 

 than the sum of AB and BC, and that the more distant 

 genes are no farther apart than indicated by the cross-over 

 percentages between them. This explanation has met with 



