PLASMAGENES AND CHROMOGENES IN HETEROSIS 235 



interpreted in other ways. The experimental evidence to date does not dis- 

 tinguish clearly between a general physiological interaction and a specific 

 contribution from favorable dominant effects. More evidence on this point is 

 needed. 



Backcrossing experiments in maize, where dominant gene markers are 

 maintained in a heterozygous condition, show heterosis continuing to the 

 sixth generation. The approach to the level of growth activity of the recur- 

 rent inbred parent is so slow as to indicate that every region of the chromo- 

 somes, divisible by crossing over, has an effect on growth. 



The growth rate in these backcrossed generations is maintained at a level 

 appreciably above the proportional number of heterozygous allelic pairs. 

 This effect can be interpreted in a number of ways other than a general 

 physiological interaction, such as enforced heterozygosity, and the comple- 

 mentary action of dominant genes at different loci. 



There is no way known at the present time to distinguish clearly between 

 the accumulation of favorable dominant effects of compound or multiple 

 genes at the same loci and a general physiological interaction or over- 

 dominance. 



Reciprocal crosses differ in many species, resulting in appreciable diver- 

 gence in the amount of growth, and these differences have a cytoplasmic basis. 

 The evidence from maize, however, shows clearly that cytoplasmic pollen 

 sterility has no effect on size of plant, time of flowering, or productiveness. 



