The Behavior of the fhromosomes as Studied through Linkage. 



249 



it is seeu that, iu general, the observed percentages in a given experi- 

 ment vary in the same direction from those found in the simpler experi- 

 ments. Exceptions to this rule are to be found in only three of the 

 eleven experiments (WMR, YWVM, and BVgCv). Of these three the 

 second is such a complex cross that no great significance is to be 

 attached to it, because of the large possibility of differential viability, 

 and tiie third is based on too small numbers to be reliable. This relation 

 seems to me to indicate that there is a real variability in strengtii of 

 linkage, aside from chance fluctuations and viability effects, even in those 

 crosses involving chromosomes I and II which give apparently normal 

 results. 



Another case showing that two pairs of genes need not always 

 show the same linkage is of course supplied by the phenomenon observed 

 in the male, where there is little or no crossing over in chromosomes 

 n and III, even for loci that cross over freely in females. 



Several cases of variations in the strength of linkage have been 

 rei)orted in other forms. Baur ('12 a) records a case in Antirrhimim 

 where two pairs of genes show no linkage, or only very little, in some 

 crosses, but give about 20" o crossing over in others. Puxnett ('13) 

 reports a similar case in the sweet pea. Two pairs of genes which 



