IN THE SECOND CHROMOSOME. 



325 



(2) This gene is not in the second chromosome at all, but in the 

 third. 



(3) The third chromosome gene is linked to a gene that is lethal 

 when homozygous. This is the reason the very high values could 

 not be fixed. 



(4) This gene, called C///, //, also causes an increase in p r c cross- 

 ing-over in Cn r females. Its effect on females of different constitu- 

 tions with respect to C H i and C n r is not yet clear. 



(5) Cj/j, j/, when heterozygous, reduces the amount of crossing- 

 over in the third chromosome. Its effect in this respect is similar to, 

 but not identical with, that of C ni (see next section, and Muller, 1916). 

 Unlike C///, it "allows" a few cross-overs between sooty and rough; 

 but it causes a reduction of crossing-over farther to the left than 

 does C ni - 



(6) Females with C///, // in one chromosome, and C in in its mate, 

 give nearly the same amount of crossing-over in the third chromo- 

 some as do females heterozygous only for C HI , or perhaps less in the 

 left-hand regions. 



A detailed comparison of the effects of these two genes, a study of 

 their interaction, and also an investigation of the locus of C IH , // are 

 now under way. 



COMPARISON WITH RESULTS OBTAINED FROM C m . 



I have shown (Sturtevant, 1913a, 1915) that great linkage variations 

 occur in the third chromosome. My own unpublished data and those 

 presented by Muller (1916) show that the case is very similar to that 

 of C// r . The factor C in , present in the beaded stock and in several 



TABLE 23. 



Total 4 high, 9 low. 



1 Probably an error in classification. Such cross-overs 

 are exceedingly rare. This individual was not tested. 



