176 



MULTIPLE FACTORS 



as one of two similar dominant factors producing a 

 non-cumulative result. (Here, then, the 9AB re- 

 semble the 3aB and 3Ab individuals and a 15:1 ratio 

 results.) To those who reject the idea that domi- 

 nance implies presence, recessiveness absence, there 

 is no great distinction between this case and that of 



CD- 



CJ- 



of CD 



CJ 



1 



CD. CD 



f 



:D . CD 



CJ . cD 



FIG. 54. Diagram showing the kinds and composition of the F 2 capsules 

 of Bursa bursa-pastoris. (After Shull.) 



the two whites with a 9 : 7 ratio. Shull found that 

 when a plant of Bursa bursa-pastoris with round 

 capsules is crossed to one with triangular capsules, 

 the round is recessive to triangular in FI. In F 2 the 

 round reappears only once in sixteen times (Fig. 54). 

 Thus in this cross round may be treated as the result- 

 ant of the two recessive factors, either of which by 



