Heterosis 387 



are crossed there will be more or less pronounced heterosis, but 

 inbreeding with selection may sometimes produce inbred lines 

 that are equal in vigor to crossbred ones. If the effective popu- 

 lation size of a species is large, harmful recessives accumulate to 

 the fullest extent possible, depending upon the mutation rates 

 of these genes. Homozygotes occur only very rarely, so poten- 

 tially harmful mutations are rarely eliminated. Large numbers 

 of such deleterious genes then accumulate in the population. In- 

 breeding will make them homozygous and, if they are not lethal, 

 the various inbred lines that contain them will survive, although 

 considerably lacking in vigor. Maize probably is to be found 

 in this group, and it is here that heterosis will be most striking 

 in its effect. 



A word might be said of the practical value of heterosis in 

 breeding maize. G. H. Shull outlined a method of breeding as 

 early as 1909 by which hybrid vigor would be utilized. It was 

 opposed as impractical by East, who advocated using commer- 

 cial strains, as had Morrow and Gardner about fifteen years 

 previously. Support for a pureline method did not come until 

 Jones's proposal of the double cross about 1917, but in spite 

 of these early discoveries, hybrid corn was still very much of a 

 novelty as late as about 1930. Ten years later, however, more 

 than 75 per cent of the commercially produced sweet corn was 

 hybrid corn, and by 1945 there were large areas of sweet corn 

 regions where no other type was grown. The best of the sweet 

 corn hybrids are produced from single crosses between two inbred 

 lines, as Shull had originally proposed; but certain practical 

 considerations in field corn breeding make a modification of this 

 method necessary. The inbred lines, themselves, are not very 

 productive and have smaller kernels than hybrids. Much of the 

 field corn is planted by machines that were constructed for larger 

 seeds. Jones's use of double crosses avoids this difficulty. Four 

 inbred lines are used, as in Fig. 101. Line B is pollinated with 

 pollen of line A, and line C with pollen of line D, producing two 

 hybrid lines, both of which show heterosis. The first hybrid is 

 then crossed by the second, and the double hybrid is used to 

 produce the maize crop. Because the hybrid used as a female 

 shows heterosis, it has large, productive ears and the large kernels 

 suitable for a corn-planting machine. 



