PLASMAGENES AND CHROMOGENES IN HETEROSIS 



229 



]y (see Table 14.1). Hybrid combinations that bring these genes together are 

 appreciably more efficient in chl()ro])hyll production than combinations that 

 lack some of them. However one of these dominant alleles has a suppressing 

 eflfect on chlorophyll development. The combination of all of these chloro- 

 phyll genes so far studied is not the most productive. There are many genes 

 of this type that block chemical syntheses, that are not lethal in the usual 

 genetic assembly, but which combine to give a cumulative efficiency in most 

 cases. 



Lethal genes which show complete dominance of the normal allele would 

 have no effect on heterosis other than to reduce the number of offspring. Such 



TABLE 14.1 



GENES CONTROLLING CHLOROPH\TL PRO- 

 DUCTION IN MAIZE* 



* Data from H. L. Everett. 



genes would be just as effective in the homozygous as the heterozygous con- 

 dition. Genes that have any part in the type of heterosis that is manifested 

 in increased growth must be viable and have some degree of dominance. In 

 other words, Aa must be greater than | AA. Aa may even be greater in 

 its effect than A A or aa in which case theoretically there is over-dominance, 

 but very little specific evidence is available to show that such a situation 

 actually exists. 



I can see no way in which it is possible to separate over-dominance from 

 a stimulus of heterozygosis. They seem to be different ways of saying the 

 same thing. The essential point at issue at the present time is whether or not 

 over-dominance actually occurs, and if so, how important this is in the 

 total amount of heterosis in addition to the known accumulation of favorable 

 dominant effects. 



INTERACTION BETWEEN ALLELES 



Evidence has been presented from many sources bearing on the problem 

 of over-dominance and interaction between different alleles. Much of the 

 argument is based on mathematical treatment of data that require many as- 

 sumptions. What is needed is more specific evidence where the effect of 



