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 
F, 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 
