Nu. 631] COLOR CLASSES IX DOVES AND CANARIES 165 



stronger claim to recognition as the correct one for explaining 

 the observed phenomena. Until such time, however, the explana- 

 tions of the observed exceptions by an hypothesis which requires 

 the appearance of an approximately equal number of exceptions 

 in the same cross, without any evidence that such exceptions exist 

 can not be considered as satisfactory. That this is the case for 

 doves has been suggested by Bridges in his more recent (1916) 

 paper on non-disjunction as already quoted. 



Non-disjunction 



In 1916 Bridges, in giving the data on which rests most of the 

 experimental proof of the existence of non-disjunction in Droso- 

 phila, suggested that the exceptions to sex-linked inheritance in 

 doves and canaries, might be the result of non-disjunction of the 

 sex chromosome. 



If, however, the mating* producing such exceptional individ- 

 uals are analyzed on the basis of non-disjunction, certain discrep- 

 ancies between expectations and actual results become evident. 

 These discrepancies suggest some fundamental difficulties in 

 applying the hypothesis of non-disjunction to the cases in ques- 

 tion. Thus if we assume as does Bridges that the sex formula in 

 both doves and canaries is FFMM in the male and FFMm in the 

 female we may represent the crosses made as follows: Theoret- 

 ically there may be either (a) no non-disjunction, or (&) non- 

 disjunction in the male, or (c) non-disjunction in the female. In 

 each mating we shall consider the three possibilities: 



1. In doves: 



In the three cases the zygotes formed will be as follows : 

 (a) If no non-disjunction: 



