Gene Interaction and Continuous Traits 



57 



creases the number of classes of F2 offspring 

 increases. When the number of classes be- 

 comes large, environmental action may cause 

 individuals to fall out of their phenotypic 

 class, so to speak, into the space between 

 classes or into an adjacent phenotypic class. 

 In this way classes become numerous, then 

 indiscrete, resulting finally in a continuous 

 range of phenotypes. 



Note also that as the number of gene pairs 

 determining the trait increases, the frac- 

 tion of all F2 resembling either Pi becomes 

 smaller. Thus, with one pair of genes V^ of 

 F2 are either black or white, with two pairs )i, 

 with three pairs ^32, etc. As a consequence 

 of this, as the number of genes increases from 

 10 to 20, etc., the continuous distribution of 

 phenotypic types forms an F2 curve which 

 becomes more and more narrow in shape. In 

 other words, the chance of recovering in 

 F2 any phenotype, a given distance off the 

 mean, decreases as gene pair number in- 

 creases. While it may be possible to identify 

 whether 1, 2, or 3 gene pairs cause a given 

 character, it is almost impossible to know 

 exactly how many are involved whenever 

 more than 3 are concerned. Nevertheless, 



a measure of how the population varies 

 relative to the average phenotype can give 

 information as to the approximate number of 

 genes involved and can be of predictive value 

 also. 



FIGURE 8-1. Dependence of number of pheno- 

 typic classes upon number of gene pairs. Horizon- 

 tal axis shows classes, vertical axis indicates relative 

 frequencies. 



1 



1 



involving one gene pair 



involving tv\^o gene pairs 



involving three gene pairs 



involving many gene pairs 



