INBRED LINES FOR HETEROSIS TESTS? 351 



important heterozygote advantage with attendant intermediate equilibrium 

 gene frequencies. 



2. Cumulative gains from recurrent selection pressure are necessary to 

 obtain efficiently crosses heterozygous for anywhere near the potential maxi- 

 mum proportion of loci, since distribution of Fi crosses within any generation 

 is narrow relative to the potential range when numbers of loci are large. 



3. Expected effectiveness of reciprocal recurrent selection between two 

 populations and recurrent selection for cross performance with a homozygous 

 tester may be compared as follows: 



a. They are alike in potential limits of cross performance for loci exhibiting 

 heterozygote advantage, but use of a homozygous tester would be more 

 likely to limit ultimate cross performance if partial dominance or special 

 epistatic effects were important. 



b. They would be similar in initial cross performance, except that it should 

 be easier to deliberately select a stock differing materially from a homo- 

 zygous tester in gene frequency at individual loci than to select two 

 equally complementary non-inbred stocks. 



c. As long as gene frequencies in the selected populations are anywhere near 

 their expected equilibria, improvement in cross performance per cycle 

 will be far greater for the homozygous tester than for the reciprocal selec- 

 tion plan. The difference between progenies from A and a gametes under 

 selection approaches zero as frequency of A in the non-inbreed tester 

 approaches an equilibrium of (Aa — aa)/(2 Aa — AA—aa), but discrimi- 

 nation between A and a gametes under selection is maximum when the 

 tester is homozygous aa or A A. 



d. Rate of progress from reciprocal selection accelerates as the difference in 

 frequency of homologous chromosomal units in the two populations be- 

 comes larger, and surpasses homozygous tester selection when qi — q^ 

 exceeds about .5. 



4. Use of a partially inbred line as one of the two populations in reciprocal 

 selection would greatly increase progress in early cycles, since individual 

 gene frequencies will be further away from equilibrium in inbred strains than 

 in non-inbred stocks. 



