354 Quantitative Characters 



number would have to be assumed, or the one particular segment 

 of a chromosome would have to have been incorporated by nu- 

 merous translocations into almost every chromosome in the or- 

 ganism. Both situations are highly unlikely. 



In Chapter 21 we mentioned that some genes exert a promi- 

 nent effect but their exact expression may be influenced in a 

 number of ways by modifying genes which would probably be 

 undetected if the gene whose character they modify were not 

 present. That such modifying genes might have an influence 

 on size cannot be doubted. It is possible, then, to explain at 

 least some quantitative characters by assuming not many poly- 

 meric genes of equal value but a number of genes no two of 

 which might produce the same effect. Three or four genes might 

 be present each of which would add considerably to size, but no 

 two would increase the size by the same amount. In addition, a 

 number of modifying genes might be present, some of which 

 would vary to a greater or lesser extent the size increment pro- 

 duced by different ones of the main size genes. Furthermore, 

 some genes might also be present which would not increase size 

 but would actually decrease it. The situation might be further 

 complicated by an interaction of the various genes such that two 

 together would produce a different result from that expected 

 merely by adding together the effects produced by both indi- 

 vidually. Although the theory of polymeric genes usually as- 

 sumes that none of the genes is dominant over its allele, it is 

 quite probable that some of the "major" genes and some of the 

 modifying genes might show dominance, some might not, and 

 some might be incompletely dominant over their alleles. Such 

 a complicated interaction of dominant and nondominant genes 

 of different values is much less easy to analyze than the simpler 

 assumption of multiple factors. 



An objection to the theory of many cumulative, polymeric 

 genes as the causative agents in size inheritance was raised 

 many years ago by Shull and Hagedoorn. Shull classified the 

 genes that determine quantitative characters into duplicate and 

 plural determiners. The duplicate genes are those which, when 

 separated from each other, produce characters so similar that 

 they cannot be distinguished from one another. They would 

 correspond to the polymeric genes or multiple factors often used 

 to account for quantitative characters. The plural determiners. 



