140 GENETICS IN RELATION TO AGRICULTURE 
full color of the parent red wheat. Consequently there are six shades 
of red in an F, population possessing various frequencies with respect 
to the proportionate number of individuals which display a particular 
shade of color as shown in the foregoing diagram (Fig. 63). Factors 
which display summation effects have been conveniently called cumu- 
lative factors. 
Besides dominant factors which produce similar or identical somatic 
effects a large number of recessive factors are known which display the 
same phenomena. The first example of this type which was worked 
out was that in sweet peas described by Bateson. In sweet peas there 
are a number of different whites which phenotypically cannot be distin- 
guished from one another. The fact that they are genetically different 
is shown when they are crossed together, for then instead of producing 
white sweet peas the F; plants bear colored flowers, the particular color 
depending upon the genetic constitutions of the whites which were 
crossed. Since the simultaneous action of two dominant factors, neither 
one of which by itself can produce any color, is necessary for color pro- 
duction, Bateson has proposed to call such factors complementary 
factors. 
The same relations have been found to exist in the production of 
aleurone color in grains of corn. Certain white varieties of corn are 
known which when crossed together give red or purple corn according 
to the genetic constitutions of the races which were crossed. As with 
dominant duplicate factors this sort of phenomenon gives peculiar 
Mendelian ratios in F2 because of the fact that many of the genotypes 
are indistinguishable phenotypically. Thus for example we may repre- 
sent a purple corn by the formula CCPP, these factors being particularly 
concerned in the production of aleurone color. A mutation in the 
locus C would give a white corn of the genetic constitution ccPP, and 
likewise a mutation in the locus P would give a white corn of the genetic 
constitution CCpp. Phenotypically these two varieties of white corn 
are indistinguishable, but from a genotypic standpoint the factors for 
white are located in different chromosomes in the two varieties. Accord- 
ingly when two such white varieties are crossed, the F; is of the genetic 
constitution CecPp. Since C and P are both completely dominant over 
their allelomorphs ¢ and p such a corn will be purple because the com- 
plete set of factors necessary for the production of purple aleurone colon 
has been brought together by crossing these two genetically different 
whites. 
The checkerboard for the Fs of such a cross is shown in Fig. 64. It 
will be observed that the phenotypic ratio in Ff, is 9 purple:7 white. 
This is merely a modification of the typical 9:3:3:1 F. ratio, for in this 
cross the last three classes are phenotypically alike, although geno- 
Digitized by Microsoft® 
