168 



potency". The „inhibiting factor", if such there be ; need not 

 even be genotypic in nature ; but may be the result of some 

 somatic quality of the parent (such for instance as its State of 

 health) projected to the offspring through influences surrounding 

 the latter during their embryonic development. 



Summary. 



Crosses between Bursa (Gapsella) bursa-pastoris simplex and 

 B. Heegeri heteris have demonstrated the existence of two genes, 

 A and B, which determine the differentiating characteristics of 

 the rosettes and which result in the production of four forms in 

 the F 2 in the ratio 9:3:3:1. 



They seem to indicate also the presence of two genes, C 

 and i) ; each of which is independently responsible for the bursa- 

 pastoris-type of capsule. The Heegeri-tyipe appears only in the 

 absence of both G and D. On this basis the two forms should 

 appear in the F 2 in the ratio 15 : 1. The observed ratio was 

 21.9 : 1. In the F 3 , some families should give ratios of 3 : 1, and 

 other families should again give ratios of 15 : 1. The observed 

 ratios in F 3 were 4.67 : 1 ; and 22.2 : 1. These observed ratios 

 show a corresponding deviation from the theoretical ratios, and 

 are thought to indicate the action of some modifying influence ? 

 such as „selective elimination", distorting the results of an other- 

 wise normal Mendelian segregation. 



A defective ratio in the rosettes of one family is shown to 

 be due to failure of dominance, and certain facts are presented, 

 which indicate that this failure of dominance is attributable to a 

 less „relative potency" of the gene A which determines the 

 ienuis- chtxrsicter. This change in the relative potency of A may 

 be due to a decline in the efficiency of the gene itself, or to 

 the Operation of some other factor or condition which offers an 

 increased resistance to the development of the £e?wis-characters. 



