174 Outline of Genetics 



hybrid vigor in this way; ''all maize varieties lose vigor 

 when inbred." 



2. Experience assures us that the distribution of 

 individuals in the F2 generation with reference to hybrid 

 vigor is represented graphically by a symmetrical curve, 

 similar to the normal probabilities curve; the class con- 

 taining the greatest number of individuals is that which 

 shows the medium amount of hybrid vigor, while on 

 either side of this class the fall in the curve is regular, 

 reaching its lowest points in the two small extreme classes 

 which show respectively greatest hybrid vigor and least 

 hybrid vigor. According to the dominance hypothesis, 

 the largest class of the F, individuals is that showing 

 greatest hybrid vigor (if only a few effective factors are 

 assumed, as was the case in the work of Keeble and 

 Pellew), while the smallest class would be that showing 

 least hybrid vigor. The curve representing such a situa- 

 tion would be asymmetrical and strikingly different from 

 that which actually occurs. 



For these two reasons the dominance hypothesis, as 

 proposed by Keeble and Pellew, has been discarded. 

 Although it is theoretically attractive, its failure to satisfy 

 these two important details of the hybrid vigor situation 

 has condemned it. 



Recently Jones (7) has ingeniously modified the 

 dominance hypothesis so as to avoid these difficulties. 

 The argument is essentially the same, with one very 

 significant modification. Jones visualizes the situation 

 as represented in fig. 27. In this case it is a question 

 of linkage of dominants and recessives. The vigor of 

 one parent is due to the two dominant factors A and D, 

 while that of the other parent is due to the two dominant 



