244 



GENETICS AND EUGENICS 



150 and 151, where they can be compared with the growth 

 curves of the respective parent races. In each case Fi sur- 

 passes either parent race in size, but F2 is intermediate be- 

 tween them. So far as heredity is concerned, the inheritance 

 is blending, but Fi shows an increase in size due to hybridi- 

 zation. It seems to be due not to heredity at all, strictly 

 speaking, but to heterosis, and it begins to disappear as the 

 Fi hybrids are bred together producing an F2 which theo- 

 retically is only half as heterozygous as Fi (Table 32a). It 

 might be expected to decline still more in later generations. 



Weight 



in\ 

 GraMs 



A^e iaDaya 40 80 120lfi0200240280320360«)a 



Fig. 150. Growth curves of race B and Cavia cutleri males and of their Fi and F2 male hybrids. 



Animal breeders have long utilized the principle of hetero- 

 sis in the production of mules and in the "grading" of cattle, 

 hogs, and sheep for meat production. Plant breeders are like- 

 wise seeking to take advantage of this same principle for im- 

 proving field crops in quantity, quality and uniformity of 

 yield. In particular East and Jones have suggested the fol- 

 lowing novel methods of breeding maize. First, a standard 

 variety should be inbred (self -pollinated) for several gen- 

 erations, in the course of which it will automatically resolve 

 itself into a number of genetically different pure lines (Fig. 

 144). Any lines inherently weak w^ill become extinct or may 

 be discarded. Those which remain will contain the best com- 

 binations of genetic factors originally present in the variety, 

 but will lack any vigor due to heterosis and so will be less 



