288 Criticism of Hypoflicsis of Linkage and Crossing Oi^er 



n — 1. If crossing over is common to homozygotes and heterozygotes, 

 as Sturtcvant suggests, then the number of crossings over, vt'hether 

 segregation takes place or not should be only one less than the numbers 

 of factors ; in other words, there must be a crossing over place between 

 each factor and its neighbours. Every possible assortment of characters 

 is thus provided for. 



But it does not seem probable that the law which applies to single 

 cross-overs should apply also to double and multiple cross-overs, and 

 indeed Morgan and his colleagues, in order to explain many remarkable 

 deviations from thi' law, have already had to resort to three or more 

 subordinate hypotheses, which they discuss in some detail under the 

 heads of (1) differential viability, (2) double or multiple crossing over, 

 (3) interference, and (4) incapacity for crossing over when the Y (male) 

 chromosome is involved ; etc. 



I have thought it worth while to trace the effect of the addition 

 of another crossing over in the case examined on pp. 284-2S6. If we 

 take the figures on pp. 284-285 numbered 1 to 4, as a starting point, and 

 confine our attention to those cmss-overs in which A is involved, we 

 get the following sets of figures and relations deduced from them. 



Uuuble Crussiiuj Over with (5 Fucturs, ABCDEF. 

 A b C D E F 



a B 



A b 



a 

 A 



ZXl 



c 

 c 



d 

 D 



e 



E 



ix^zyz 



zxz 



B C D e 



b c d e 



f 



F 



a B C d e f 



A b c d E F 



f 



F 



B 



