222 MULTIPLE FACTORS 



having two pairs of recessive factors for white is 

 crossed with a colored sweet pea (AB)^ it is found 

 when the 9AB: 3aB: 3Ab: lab individuals appear in 

 F2 that the aB and Ab plants, having only one 

 factor for white and one for red, are just as white as 

 the ab plants. In other words, the ab class can show 

 no cumulative effect of the two white factors. Since 

 the three latter classes all look white, they are added 

 together in the count, and a ratio of 9 reds : 7 whites 

 results. 



It is commonly said that this result is due to the 

 occurrence of two factors ''for red" (the dominants, 

 A and B), neither of which alone is sufficient to 

 produce any effect (since Ab and aB look no different 

 from ab), but which, when present together, act as 

 complements to each other and thus produce the red 

 color. Such an interpretation fails, however, to take 

 into consideration the possible effects of the recessive 

 factors ''for white" (a and b). It is therefore un- 

 warranted, unless the "presence and absence" view 

 be accepted, namely, that the dominants are the 

 only real factors, the recessives being mere absences. 

 It would likewise be unwarranted, of course, to 

 ascribe the results purely to the recessive factors, and 

 so to conclude the similarity of aB and Ab to ab was 

 due to the fact that a and b were non-cumulative in 

 their effects. Neither of these methods of describing 

 such cases is therefore to be regarded as more than a 

 shorthand statement of the empirical facts. 



In the cross of Bursa which follows, Shull, using 

 the presence and absence scheme, treated the case 



