330 



INHERITED LINKAGE VARIATIONS 



Table 25. 



'^ S' b -\- b Pr -\- Pr c '\' c Sp, except in the last two columns, where pr Sp 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. 

 Cm, located somewhere to the left of purple, decreases the amount 

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

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

 crossing-over between purple and speck in females heterozygous for 

 it; but females homozygous for Cur 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 Cur 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. 



