486 



JOHN W. GOWEN 



definitely differs genetically and otherwise from that of the flies which pro- 

 duce even one egg in their lifetimes. For this reason and the fact that hetero- 

 sis is a phenomenon of quantitative inheritance, we have excluded such flies 

 from consideration in these studies. 



The general analysis of the variations within these hybrid egg yields is 

 presented in Table 29.5. 



TABLE 29.5 

 DISTRIBUTION OF VARIANCE IN EGG PRODUCTION 



100.0 



Table 29.5 presents data on the factors which may be of importance in the 

 interpretation of heterotic effects. The percentage contribution of each factor 

 is shown in the right hand column. The largest contribution, residual, is made 

 by the variation within flies of a given age. It is half of the total observed. 

 This variation shows what minor differences in seemingly constant condi- 

 tions can be responsible for differences in egg yields. 



Differences in races represent the next most significant contribution to 

 yield variations — 15 per cent. Major contributor to this effect is the differ- 

 ence between the productivities of the inbred parent races and their hybrids. 

 These differences may be looked upon as the effects of additive genetic fac- 

 tors for yield, and the effects of specific gene combinations leading to the ex- 

 pression of dominance, overdominance, or epistacy in the phenotypes. 



A point of currently even more importance brought out by these data is 

 the dependence of yield on the close interrelation of environment and geno- 

 type. The interaction of experiment X race accounts for 9.8, and experiment, 

 race, and age, 3.1 per cent of the variation. The total is 12.9 per cent. Even 

 with great care to closely control conditions both within and between experi- 



