PLASMAGENES AND CHROMOGENES IN HETEROSIS 231 



cob, yellow endosperm, and non-glossy seedlings— were chosen because Ihey 

 had little or no effect on growth of the plant. 



The first generation outcrosses showed the usual large increases in size of 

 plant, time of flowering, and yield of grain that is expected in crosses of un- 

 related inbred strains of corn. The hybrid plants were backcrossed as seed 

 parents with pollen from the inbred with the recessive gene marker. In every 

 generation, plants with the dominant gene marker were selected for back- 

 crossing. These plants have now been backcrossed six successive times. Many 

 progenies have been grown. They are all heterozygous for the gene marker 

 plus whatever neighboring regions on the same original chromosome from 

 the non-recurrent parent that have not been lost by crossing over. 



The plan is to continue the backcrossing until no measurable differences 

 remain between the backcrossed plants and the recurrent parent, or be- 

 tween the two classes of backcrossed individuals in the same backcrossed 

 progeny, those with the dominant marker and those with the recessive 

 marker. When the point is reached where no differences can be detected, the 

 plan is to compare successive earlier generations from remnant seed to pick 

 up whatever single gene differences there might be that could be measured 

 and detected by their segregation. 



So far both classes of backcrossed plants in nearly all progenies are taller 

 and flower earlier, showing that they have not been completely converged to 

 the parental type (see Table 14.2). The differences are small and not statisti- 

 cally significant in the tests so far made, but are nearly all in the direction of a 

 heterotic effect. As yet there are not sufiicient data to base final conclusions. 

 It is hoped that the comparison of the two classes of backcrossed progeny 

 with the original recessive parent will permit a distinction between the favor- 

 able action of dominant genes and an interaction between heterozygous 

 alleles. Also that it may be possible to show whether or not there is any 

 residual cytoplasmic effect, since some of the outcrossed plants have the same 

 cytoplasm as the dominant gene marker and some do not. 



Important facts do stand out clearly from this experiment. Since heterosis 

 still remains after these many generations of backcrossing, it shows clearly 

 that these three chromosome regions selected as samples have an appreciable 

 effect on growth. Since the gene markers themselves have no effect on 

 growth, as far as this can be determined in other material, these three regions 

 are random selections for growth effects. This indicates quite clearly that 

 there are genes in all parts of the chromosomes that contribute to normal 

 growth and development. While the evidence so far available does not per- 

 mit a clear separation between the effects of an accumulation of favorable 

 genes as contrasted to an interaction between alleles, or between genes and 

 cytoplasm, the results show that there are many loci involved in the heterotic 

 effect in addition to the dominant gene markers. 



This follows from the evidence at hand. If the heterosis now remaining 



