180 



THE THEORY OF THE GENE 



In their study of the trisomic types of Datura, Blakes- 

 lee and Belling have found about 12 distinct types be- 

 longing to the 2n4-l or trisomic series. Since there are 

 just 12 pairs of chromosomes, only 12 simple trisomic 

 types are expected, and, in fact, evidence has been found 

 that there are only 12 such primary types. The rest, 

 called secondaries, appear to belong to one or another of 

 the 12 primary types (Fig. 102). The evidence for this 

 comes from several sources, from similarities in external 

 appearance, from internal structures (as shown by Sin- 

 nott), from their similar mode of inheritance (giving the 

 same trisomic inheritance for marked chromosomes), 

 from the reciprocal throwing of one member of the group 

 by the other, and from the sizes of the extra chromosomes 

 (Belling). 



In the following table a list of the primaries and their 

 secondaries is given. These have been derived from trip- 

 loids. 



Primary and 

 (Primaries 



Secondary (2n+l) Types in Offspring from Triploids 

 are printed in capitals, secondaries in lower case type.) 



