182 THE THEORY OF THE GENE 



spine factors A, a, indicating that both Poinsettia and 

 Wiry have their extra chromosomes in the set carrying 

 the factors P, p, but not in the set with the factors A, a. 

 Similarly, tlie ratios for Cocklebur indicate that this Pri- 

 mary has its extra chromosome in the set carrying the 

 factors A, a, but not in the set with factors P, p. Its 

 Secondary Wedge, however, fails to give trisomic ratios 

 for A, a. The ratios actually found resemble those in 

 disomic rather than in trisomic inheritance and seem to 

 indicate a deficiency in the extra chromosome of Wedge 

 for the locus A, a, since the evidence strongly indicates 

 that it is a Secondary of Cocklebur. If A' indicates the 

 modified chromosome and A and a go to opposite poles 

 at reduction division in a Wedge plant with the formula 

 AA'a, the gametes would be A+a+AA'+aA'. Such be- 

 havior would account for the ratios [in table 5]. If A' is 

 deficient for the factor A, the gamete a A' would carry no 

 factor for A ; hence the disomic ratios between armed and 

 inermis Wedges found but not sho^^ni in the table. If A 

 and a, occasionally should go to the same pole, the 

 gametes would be A' (which would probably die) and Aa, 

 which would go to form a Primary Cocklebur occasion- 

 ally thrown by Wedges. 



''The hypothesis of a deficiency in the extra chromo- 

 some of Secondaries has been strengthened by Dr. Bell- 

 ing 's cytological findings. His hypothesis of reversed 

 crossing-over, however, completes the picture by indicat- 

 ing a doubling of a part of the chromosome along with a 

 deficiency of the remaining portion." 



Tetraploids of Datura with an additional chromosome 

 have also been reported (Fig. 103). In one of these sho^vn 

 in the figure there are five like chromosomes in one group, 

 and in the other there are six like chromosomes. 



Belling and Blakeslee have studied the modes of union 

 of the three chromosomes in the primary and in the sec- 



