HEREDITY AND EVOLUTION 245 
the different factors were cumulative in their action, and that 
all four factors were necessary for the production of the depth 
of color shown by the parent red strain. If we let 4 stand for 
one of these factors for 
red, and B for the other 
factor, then the red strain 
may be represented by 
AABB, and the white 
strain by aabb. When 
these two strains were 
crossed, the /’, genera- 
tion contained the factors 
Aabb. The individuals 
of this generation thus 
contained only half as 
many factors for red as 
the parent red_ strain, 
and their color was in- 
termediate between that 
of the red and the white 
strain. When this /, gen- 
eration formed gametes, 
they were of four kinds, 
EaieeeAv, ab, and ab. 
The F, generation should 
therefore show the six- 
teen combinations char- 
acteristic of dihybrids. 
Fig. 237 shows the re- 
sults. An examination 
of the squares in the 
lower part of the diagram 


R 
I 
3 a bis apbDis aADhe 
Fig.237. Diagram representing inheritance 
of two pairs of cumulative factors, AA and 
BB, which produce a red color in wheat 
The absence of A and bis represented by a and 
brespectively. Squares represent individuals ; 
circles, gametes. The number of factors for 
red, and consequently the degree of redness, 
is shown by the number of the letters R in the 
wheat grains - 
shows that only one out of sixteen does not contain either A: or 
B. This individual has only recessive characters and is white, 
while the other fifteen are red. Only one, however, contains 
four factors for red, and this is the only one with as deep a 
