CROSSING-OVER IK HYBRIDS 



287 



it occurs in one arm the reduction is stronger in the affected arm, 

 while in the unaffected arm it is normal. Further, Dobzhansky 

 has shown that this reduction is due to the interference of the 

 newly associated segments with one another in pairing, for when 

 a long segment of an autosome is translocated to the Y chromo- 

 some (which has no partner, at least for the greater proportion of 



55Xta./ 4t. 



58Xta /5t 



Fig. 95. — The twelve diplotene bivalents in two clones of Fritillaria 

 imperialis, giving the total number of chiasmata and the number 

 terminal in each. The upper one has three fragments united by 

 a triple chiasma ; the lower one has ten fragments, only two of 

 which are paired. Cf. Table 17 and Fig. 48 (from D., 1930 c). 



its length), its crossing-over is not sensibly reduced. There is no 

 interference. When the translocated segment is short, its crossing- 

 over is strongly reduced. Evidently, therefore, the chance of two 

 homologous segments pairing is decreased when the segment is less 

 than a certain length. Since the fourth chromosome is shorter 

 than any segment concerned, and yet pairs fairly regularly, this 

 decrease is clearly due to the change of association. 



