246 A TEXTBOOK OF GENERAL BOTANY 
color as the original red strain. Six individuals contain two 
factors for red, or exactly half the number in the original red 
strain. The number of factors for red (and so the depth of 
color) is indicated by the number of the letters # shown in the 
grains in the squares. 
Another set of experiments with wheat is even more interest- 
ing in connection with cumulative factors. A different red 
strain was crossed with a white strain, and the individuals of 
the F generation were again intermediate in color. In the F, 
generation there were sixty-three reds of various shades to one 
white. In this case the red strain appeared to contain three 
pairs of factors for red, all of which seemed to produce about 
the same degree of color and all of which were necessary to 
produce the depth of color shown by this stram. ‘The propor- 
tion of sixty-three red to one white is evidently a trihybrid 
ratio. In trihybrid ratios there is only one out of sixty-four 
in the F, generation which is purely recessive, and so, if the 
red strain contained three pairs of factors for red, the F, gen- 
eration of a cross between this red and a white should produce 
only one out of sixty-four individuals without a factor for red. 
Likewise, there would be only one out of sixty-four which 
would have six factors for red and which would be as dark 
as the red strain. In all there were six degrees of redness. 
In some cases crosses are made in which offspring interme- 
diate in characters appear to breed more or less true to the 
type of the #, generation. It is interesting to consider these 
cases in connection with cumulative factors. If there were six 
pairs of cumulative factors, then in the #, generation each of 
the original types would be represented by only one individ- 
ual out of 4096. The F, generation would be intermediate in 
characters between the two parent forms, and in subsequent 
generations it would be very rare to find an individual like 
either of the parent types; unless very large numbers of indi- 
viduals were reared, such types might fail altogether to appear. 
The hybrid would then seem to breed more or less true to the 
hybrid type. 
