236 



CHAPTER 27 



applied to agriculture, since it has been esti- 

 mated that the use of hybrid corn alone has 

 already enriched society by a billion dollars. 

 You might ask: What is wrong with normal 

 corn? The answer is that it is too variable 

 in quality and vigor (Figure 27-4). The 

 variability can be decreased by inbreeding, 

 but unfortunately this also results in loss of 

 vigor or other desirable traits. The way to 

 overcome this impasse is first to obtain 

 inbred lines, which are uniform because they 

 are homozygous and which carry different 

 desirable dominant genes (yet are also homo- 

 zygous for different undesirable recessive 

 genes). If different inbred lines are crossed, 

 the Fi will be multiply heterozygous, uni- 

 form, and more vigorous than either parental 

 inbred line. 

 Accordingly, hybrids can be made be- 



tween two selected inbred lines. But while 

 these Fi plants are vigorous and uniform, 

 they come from seeds produced on ears 

 grown on one of the less vigorous inbred 

 lines. For this reason such seeds are not suf- 

 ficiently abundant to make it economical 

 to use them in raising crops of corn. This 

 difficulty is overcome in practice (Figure 

 27-5) by first making two different single 

 cross hybrids by crossing four selected 

 inbred lines two by two. Then the two 

 hybrids are crossed. Seeds produced by this 

 double cross are plentiful, since they are 

 formed on a vigorous single cross hybrid 

 plant, and can be sold inexpensively to 

 farmers for planting. Breeding procedures 

 resulting in hybrid vigor are applied to many 

 plants and animals; heterosis is of great eco- 

 nomic importance. 



SUMMARY AND CONCLUSIONS 



Two of the factors which decide the fate of a mutant in the gene pool are mutation and 

 selection. For equal mutation rates, the lower the reproductive potential the mutant causes, 

 the lower is the frequency of the mutant in the gene pool. Specific examples of rare mutants 

 are described which lower reproductive fitness by being dominant lethal, dominant detri- 

 mental, recessive lethal, and recessive detrimental. 



Nonrandom breeding, by assertive mating or inbreeding, increases the rate of homozy- 

 gosis. The per-generation rate of reduction in heterozygosity due to inbreeding is Vi for 

 self-fertilization, % for sib matings, % for half-sib matings, and Ke for cousin matings. 

 Homozygosis in normally cross-fertilizing individuals leads to loss of vigor, while heterozy- 

 gosis is accompanied by heterosis, or hybrid vigor. 



Heterosis is the phenotypic result of gene interaction, and occurs because the heterozygote 

 either serves to mask different detrimental recessives which are homozygous in the parents, 

 or is adaptively superior to both types of homozygote. Heterosis is of great economic im- 

 portance. 



REFERENCES 



Allison, A. C, "Sickle Cells and Evolution," Scient. Amer., 195:87-94, 1956. 



Crow, J. P., Genetics Notes, 4th Ed., Minneapolis, Burgess, 1960. 



Dobzhansky, Th., Evolution, Genetics and Man, New York, John Wiley & Sons, 1955. 



Dobzhansky, Th., Genetics and the Origin of Species, 3rd Ed., New York, Columbia Uni- 

 versity Press, 1951. 



Gowen, J. W. (Ed.), Heterosis, Ames, Iowa, Iowa State College Press, 1952. 



