174 Plant Genetics 



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 domi- 

 nance hypothesis the largest class of the F2 individuals 

 would be that showing greatest hybrid vigor, while 

 the smallest class would be that showing least hybrid 

 vigor. The curve representing such a situation would 

 be unsymmetrical and strikingly different from that 

 which actually occurs. 



For these two reasons the dominance hypothesis 

 seems to have been discarded. Although it is theoreti- 

 cally attractive its failure to satisfy these two important 

 details of the hybrid vigor situation has condemned it. 



Very recently Jones (4) has ingeniously modified the 

 dominance hypothesis so as to avoid these difficulties. 

 At first consideration his theory seems to be clearly the 

 most reasonable explanation of hybrid vigor that has 

 yet been presented, although in time it may suffer 

 from destructive criticism. The argument is essentially 

 the same as that for the old dominance hypothesis, with 

 the following important modification. Under the old 

 hypothesis it was stated that in the cross AAbb X aaBB = 

 AaBb the hybrid showed vigor because it combined 

 the two dominant determiners AB. This met with the 

 practical objections mentioned above. Jones visuaUzes 

 the situation as represented in fig. 38. In this case it is 

 a question of linkage. The vigor of one parent is due 

 to the two dominant determiners A and D, while that 

 of the other parent is due to the two dominant deter- 

 miners C and B. The hybrid is more vigorous than 



