140 MENDELISM chap. 



first-named is dominant. When a black fly with 

 vestigial wings is crossed with a grey normal the 

 offspring are all grey normals, the result being the 

 same whether the black vestigial is used as the male 

 or the female parent. 



When the F^ ^ is mated back with the double 

 recessive, the black vestigial female, he produces 

 only two types of offspring, viz. grey normals 

 and black vestigials in equal numbers. This is 

 what would naturally be expected on the chromo- 

 some theory in its simplest form. For of the two 

 chromosomes belonging to the second pair that 

 he contains, one was derived from a grey normal, 

 and the other from a black vestigial parent. Conse- 

 quently, when these chromosomes separate again 

 in the formation of the gametes we should expect 

 half of these to contain the grey normal chromo- 

 some, and the other half the black vestigial one. 

 The result of crossing the F^ ^ back with the 

 double recessive clearly indicates that this is what 

 happens. The factors for grey and normal wing 

 remain together, thus conforming to the view that 

 the integrity of the chromosome, in which both occur, 

 is preserved. 



When, however, we turn to the behaviour of the 

 F^ 9 we find a different state of affairs. The 

 result of crossing her with the double recessive, 

 the black vestigial male, is to lead to the production 

 of four kinds of offspring, viz. grey normal, grey 

 vestigial, black normal, and black vestigial. The 

 two classes grey normal and black vestigial are 

 about four times as numerous as the other two 

 classes, grey vestigial and black normal. Evidently 



