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CHAPTER 15 



figure 15-6. The variability of normal corn is pointed out by James F. Crow. 

 {Photographed in 1959 by The Calvin Company.) 



within the inverted region, this adaptively 

 favorable gene content tends to remain in- 

 tact in the heterozygote because of the fail- 

 ure of single crossovers within the inverted 

 region to enter the haploid egg nucleus. 



Breeding procedures that result in hybrid 

 vigor have been widely applied to econom- 

 ically important plants and animals. For 

 example, it has been estimated that the use 

 of hybrid corn has enriched society by more 

 than a billion dollars. We might ask: What 

 is wrong with normal corn? The answer is 

 that it is too variable in quality and vigor 

 (Figure 15-6). Inbreeding decreases vari- 

 ability, but unfortunately inbreeding also re- 

 sults in loss of vigor or other desirable 

 traits. The way to overcome this problem 



is to obtain inbred lines which are uniform 

 (because they are homozygous) and carry 

 different favorable dominant genes (yet are 

 also homozygous for various undesirable re- 

 cessive genes ) , and cross the different inbred 

 lines to each other. Their Fi will be multi- 

 ply-heterozygous, uniform, and more vigor- 

 ous than cither parental inbred line. 



Consequently, hybrids are made from two 

 selected inbred lines — of corn in this case. 

 Although the Fi plants are vigorous and 

 uniform, they come from kernels grown on 

 one of the less vigorous inbred lines. For 

 this reason, hybrid seeds are not sufficiently 

 numerous, and consequently, commercially 

 unfeasible. In practice this difficulty is over- 

 come (Figure 15-7) by crossing four se- 



