WHEAT, OATS, BARLEY 19 



To the untrained plant experimenter it would 

 have appeared that this experiment should be 

 carried no further. Progress was apparently 

 being made in the wrong direction; for whereas 

 half the parents were immune to rust, all of the 

 children were susceptible. 



But Professor Biffen knew, as we have 

 already seen, that susceptibility and immunity 

 constituted a Mendelian pair of hereditary fac- 

 tors. So he knew that in the next generation 

 one-fourth of the hybrid plants would be immune 

 to rust. And this expectation was justified by 

 results. The second generation hybrids showed 

 diverse combinations of various other qualities 

 that were under consideration, and a certain pro- 

 portion of them revealed the combination of the 

 desired quality of grain with the stems immune 

 to the attacks of the rust fungus. 



As immunity to rust is a recessive factor, it 

 follows that the second generation hybrids that 

 show such immunity will breed true to that char- 

 acter. Their offspring will be immune. But as 

 regards certain other qualities, notably hardness, 

 it was necessary to continue the experiment 

 through a third generation, in order to discover 

 which of the plants that were individually hard 

 were pure dominants as regards the quality of 

 hardness. 



