350 GORDON E. DICKERSON 



SUMMARY 

 Genetic Variability in Economic Characters of Swine 



1. Inbreeding and crossbreeding effects indicate that degree of hetero- 

 zygosity exerts a major influence on the important performance characters, 

 and that a high degree either of dominance or of epistacy due to deviations 

 from an optimum genetic intermediate, or both, characterizes genetic vari- 

 ability in performance. 



2. Relative ineffectiveness of selection within mildly inbred strains makes 

 ordinary dominance or epistasis doubtful as an explanation of inbreeding de- 

 cline, and suggests heterozygote advantage for net desirability in prolificacy, 

 suckling ability, viability, and growth rate. 



3. "Controlled" selection experiments with swine show that high and low 

 lines for growth rate or feed utilization can be separated, but indicate little 

 improvement of high line over foundation stock, particularly for net per- 

 formance in all characters. 



4. Lower heritabilities and larger inbreeding declines for characters long 

 and intensely selected in one direction, compared with those selected toward 

 an intermediate or in varying directions, indicate a higher degree of domi- 

 nance for the former. 



5. Some sort of negative relationship between components of total per- 

 formance is indicated by lower heritability for total performance than for its 

 component characters and by direct estimates of correlation. This would 

 correspond to heterozygote superiority, in that increased frequency of genes 

 with dominant favorable effects on one character would constitute decreased 

 frequency of their alleles having dominant favorable effects on other char- 

 acters. 



6. The genetic basis of performance appears to be similar in corn and in 

 swine, as indicated by natural degree of inbreeding, extent of inbreeding 

 decline in performance, and the effectiveness of phenotypic selection. Ordi- 

 nary dominance is inadequate to account for heterosis already achieved in 

 corn, and by analogy, is unlikely to be adequate in swine. 



7. Examples of manifold effects and heterozygote advantage for specific 

 chromosome segments or loci support their inferred importance for quantita- 

 tive economic characters. 



Methods of Selecting for Maximum Heterosis 



1. Intensity of selection per unit of time is lower when based on progeny 

 performance in test-crosses than when based on individual and family per- 

 formance. Hence, methods of selecting for maximum cross performance be- 

 tween complementary strains are indicated only when individual and family 

 selection have become relatively ineffective, and when there is evidence for 



