198 



T. H. MORGAN. 



This result is what is to be expected on the basis of no crossing 

 over in the heterozygous male as the following analysis shows: 



Black vestigial 9 , 



bVy bVg 



BVgbVg 



bi' g BV g , gray long. 



bv g bvg, black vestigial. 



The FI heterozygous females (sisters to the males just tested) 

 were in turn mated to black vestigial males and gave: 



These results are explicable if crossing over (17.1 per cent.) 

 takes place in the FI heterozygous females, as indicated below: 



Fi, 



Black vestigial, 



bV a BV a bv a Bv a 



bV g bv g , black long. 



BVg bv g , gray long. 



bv g bv g , black vestigial. 



Bv g bvg, gray vestigial. 



It has been pointed out that when the factors for black and for 

 long wings go in together, i. e., enter with the same chromosome, 

 and the factors for gray and for vestigial enter with the other 

 chromosome (it does not matter which parent contributes each) 

 the double recessive does not appear in Fo. In more general 

 terms it may be said, for the second chromosome, that when two 

 recessives enter, one from each side, the double recessive does not 



