30 



BREEDING CROP PLANTS 



intermediate densities. The determination of just how many 

 factors were involved could not be made without a more extensive 

 test. The results can be explained on a genetic basis by the 

 hypothesis that Zeocriton contains three independently in- 

 herited factors for density and that Hanna lacks these factors. 

 The added hypothesis may be made that each factor in a hetero- 

 zygous condition gives half as great an effect as when homo- 

 zygous. The factors may be considered to have a cumulative 

 effect, two factors when present in a hornozygous condition 



1.4 1.61.82.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3-6 3-8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 

 la MM 



FIG. 9. Diagrams showing the densities of parental forms and of the F 

 generation in a cross between the Zeocriton and Hanna barleys (upper), of four 

 pure lines (middle), and of several heterozygous lines (lower). (After Hayes and 

 Harlan, 1920.) 



producing twice as great an effect as when a single factor is 

 hornozygous. Other factors of a smaller value are also doubtless 

 present which modify the expression of the main density factors. 

 East and Jones have summarized the results of such controlled 

 crosses and they find a number of general conditions fulfilled. 



"1. When pure or homozygous races are crossed, the FI populations 

 are similar to the parental races in uniformity. This conclusion 

 devolves from observations that if any particular factors AA and aa 

 are homozygous in the parental races, they can only form Aa individuals 

 in the FI generation. 



"2. If the parental races are pure, F 2 populations are similar, no 

 matter what FI individuals produce them, since all variability in the 

 FI generation is the result of varying external conditions. 



