EXPERIMENTAL STUDY OF HEREDITY 71 



having a low yield per acre, are crossed, there results an 

 FI hybrid progeny that is heterozygous for all of these 

 characters. This heterozygosity is correlated with a 

 greatly increased vigor; the plants are much larger, and 

 the yield per acre is enormously increased (Fig. 43). 

 Thus in one experiment of this kind the average yield of 

 the heterozygous horticultural variety was 61.25 bushels 

 per acre. After self-fertilization for several generations 

 the yield became reduced to 29.04 bushels per acre; but 

 in the FI generation of a cross between two of these self- 

 fertilized strains the yield per acre rose at once to 68.07 

 bushels. In the F 2 generation the yield again fell to 44.62 

 bushels. From this, and numerous other experiments, it 

 is found that the biggest corn crop is to be obtained by 

 finding the strains that will produce the largest yield 

 when crossed, and then using for seed the grains of the 

 first-generation hybrids each year. 



62. Breeding for Disease-resistance. Biffen, in 

 England, crossed a wheat of poor quality, but resistant 

 to rust disease (Puccinia glumarum), with a superior 

 variety but very susceptible to the disease. Suscepti- 

 bility proved dominant in the FI generation, but in the 

 F 2 generation disease-resistant forms appeared, of superior 



FIG. 43. Zea Mays. In the experiment, the results of which are here 

 illustrated, nine strains of Indian corn were selected according to the 

 number of rows of kernels on the cob, varying from 8 to 24 rows. These 

 were pollinated by hand each year, with mixed pollen, in such manner that 

 self-pollination was entirely prevented. An average ear of each strain is 

 shown in the first row above. In the second row is shown an average 

 ear of each strain after self-fertilization for five generations. Note the 

 resulting decrease in the number of rows, lack of filling out of the ears, 

 and other marks of inferiority. The last row shows the remarkable and 

 immediate increase of vigor resulting in the FI generation of hybrids be- 

 tween various pairs of the selfed strains. (Photo supplied by G. H. Shull.) 



