The Theory of Polymery 



347 



to produce a trait that shows continuous variation, is a fascinat- 

 ing speculation. The important question, however, is whether 

 this theory can actually be used at least as a working hypothe- 

 sis to explain actual cases of continuous variation. It has been 

 applied by a number of investigators in the field of genetics to 

 many different characters in plants, human beings, and other 

 animals and, at least for some quantitative characters, seems to 

 be a reasonable working assumption even though, as some others 

 claim, it may not actually represent the facts. Let us construct 



TABLE 18 



F2 Family from a Cross between a 54-cm Plant and a 30-cm Plant Which 



Differ by Twelve Polymeric Genes 



(Each contributing gene adds 2 cm to the height of the plant.) 



a more complicated situation and see how it would work out. 

 Let us assume that two plants differ by six loci or a total of 

 twelve polymeric genes. Let us assume that they share a residual 

 heredity of 30 cm and that each contributing gene adds 2 cm to 

 height. A plant 54 cm tall of the genotype T^T^ T2T2 T^T^ T^T^ 

 T5T5 TqTq XX is crossed with one only 30 cm in height and of 

 the genotype ^1^1 ^2^2 ^3^3 Uti t^U IqIq XX. The Fi has one con- 

 tributing gene from each locus and is therefore 42 cm tall. All 

 the Fi plants would be alike genotypically so that any pheno- 

 typic differences between them would be purely environmental. 

 The Fi as a whole would show little variability, and the same 

 would be true of the two homozygous lines from which the 

 parental plants were taken. When any Fi plant was selfed it 

 would produce an F2 family which would theoretically segregate 

 as is shown in Table 18. Theoretically each parental extreme 

 would be recovered; but 4096 would be the theoretically mini- 

 mum number of plants that would have to be raised to obtain 

 one plant of each parental type. The average height of an F2 



