330 



INHERITED LINKAGE VARIATIONS 



TABLE 25. 



1 S' b + b p r + p r c + c Sp, except in the last two columns, where p T s p is used. Probably 

 all but the second and fifth columns are too low, since no correction has been made for unob- 

 servable double cross-overs. 



SUMMARY. 



Two genes that affect the amount of crossing-over in the second 

 chromosome are discussed. Females of various constitutions with 

 respect to these genes give the results shown in table 25 and figure 1. 

 C//j, located somewhere to the left of purple, decreases the amount 

 of crossing-over between star and purple in females heterozygous for 

 it. Cj jr , located between purple and speck, reduces the amount of 

 crossing-over between purple and speck in females heterozygous for 

 it; but females homozygous for C IIr show the usual amount of 

 crossing-over. 



Neither of these genes causes any change in the usual condition of 

 no crossing-over in males. 



An incompletely investigated case of increased crossing-over be- 

 tween purple and curved is apparently due, in part at least, to a 

 dominant third-chromosome gene. 



A cross-over gene, located in the third chromosome, affects that 

 chromosome in much the same way that C// r affects the region in 

 which it lies. 



In all these cases the amount of crossing-over is changed, often 

 markedly so. But the sequence of the genes is unchanged ; and females 

 of any one constitution give as consistent results as do " normal" 

 females. 



