IN THE SECOND CHROMOSOME. 



325 



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

 third. 



(3) The third chromosome gene is Unked to a gene tluit is h'lluil 

 when homozygous. This is the reason the very iiigh valuen could 

 not be fixed. 



(4) This gene, called Cm, ji, also causes an increase in p, c croas- 

 ing-over in C/j^ females. Its effect on females of dilTorcnt constitu- 

 tions with respect to Cm and Cur is not yet clear. 



(5) Cjii, JI, when heterozygous, reduces the amount of cro^King- 

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

 but not identical with, that of Cm (see next section, and MuUer, IDUi). 

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

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

 does Ciw 



(6) Females with Cnii ii in one chromosome, and Cm in its mate, 

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

 some as do females heterozygous only for Cm, 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 Cm, n are 

 now under way. 



COMPARISON WITH RESULTS OBTAINED FROM Q 



/;■ 



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

 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 My own unpublished data and those 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 diT- The factor Cm, present in the beaded stock and in several 



Table 23. 



Total — 4 high, 9 low. 



» Probably an error in cla.s.sification. Surh rr<):'.-«-ov.T.< 

 are exceedingly rare. This individual wa.s not tested. 



