MULTIPLE FACTORS 221 
with reference to each other, but by their different 
linkage values with other factors. 
An example of a cross, involving at the same time 
two factor-differences which have similar effects, is 
Nilsson-Ehle’s cross of dark brown oats having two 
dominant factors for dark glumes with white-glumed 
plants having the two recessive allelomorphic factors 
for light color. The expected F, ratio is 9 double 
dominant dark browns (AB): 3 light browns having 
the first recessive and the second dominant (aB): 3 
light browns having the first dominant and the 
second recessive (Ab): 1 double recessive white (ab). 
Since the two factor-differences produce similar 
results, however, the light browns, aB and Ab, are 
indistinguishable; counting these two classes to- 
gether, a9:6:l ratio results. The 9 double dominants 
were distinguishable from the 6 single dominants, 
the pigment being dark brown in the 9 cases where 
both factors for dark glumes were present and both 
factors for light glumes absent, but only light brown 
in the 6 cases where one light and one dark factor 
were present. Similarly the 1 double recessive, 
having both light and no dark factors, was much 
lighter even than the 6 light browns. This result 
may be described by saying that the effects of the 
factors for dark and for light were all cumulative 
or summative, two darks producing a blacker pig- 
ment than one, and two lights a paler color than one. 
In many cases, multiple factors do not give results 
that may, in the above sense, be called cumulative. 
For example, if a white-flowered sweet pea (ab) 
