140 GENETICS IN RELATION TO AGRICULTURE 



full color of the parent red wheat. Consequently there are six shades 

 of red in an F 2 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 Fi 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 F 2 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 Fi is of the genetic 

 constitution CcPp. Since C and P are both completely dominant over 

 their allelomorphs c and p such a corn will be purple because the com- 

 plete set of factors necessary for the production of purple aleurone coloi 

 has been brought together by crossing these two genetically different 

 whites. 



The checkerboard for the F 2 of such a cross is shown in Fig. 64. It 

 will be observed that the phenotypic ratio in F 2 is 9 purple:? white. 

 This is merely a modification of the typical 9:3:3:1 F 2 ratio, for in this 

 cross the last three classes are phenotypically alike, although geno- 



