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DONALD F. JONES 



cytoplasmic difference has no appreciable effect on size of plant as measured 

 by height at the end of the season, in days to silking, or in yield of grain. 

 The results are given in Table 14.3. With respect to pollen sterility-fertility, 

 the cytoplasm has no effect on heterosis. 



In the conversion of standard inbreds to the cytoplasmic sterile pollen 

 condition, it has been found that many of these long inbred strains, presum- 

 ably highly homozygous, are segregating for chromogenes that have the abil- 

 ity to restore pollen fertility. In normally fertile plants these genes have no 

 way of expressing themselves. They are not selected for or against unless 

 they contribute in some way to normal pollen production. It is one more 



T.\BLE 14.3 

 A COMPARISON OF FERTILE AND STERILE MAIZE PLANTS 



source of evidence to show that there is a considerable amount of enforced 

 heterozygosity in maize. Even highly inbred families remain heterozygous. 

 This has been shown to be true for other species of plants and animals. 



SUMMARY 



Specific evidence from a study of chlorophyll production in maize and 

 from similar studies in Neurospora, Drosophila, and other plant and animal 

 species proves conclusively that there are numerous mutant genes that re- 

 duce the ability of the organism to grow and to survive. Such genes exist 

 in naturally self-fertilized and cross-fertilized organisms and in arti- 

 ficially inbred families such as maize. The normal alleles of these mutant 

 genes show either complete or partial dominance, and any crossbred indi- 

 vidual contains a larger number of these dominant, favorable alleles than 

 any inbred individual. 



Evidence from Nicotiana shows that there is an interaction between di- 

 vergent alleles at the same locus such that the heterozygote produces a larger 

 amount of growth and a higher reproductive rate than either homozygote. 

 There is no increase in growth rate and this instance is considered not to be 

 heterosis. The assumption of an increased growth rate, or true heterosis, in 

 such allelic interactions is not supported by specific evidence that cannot be 



