Genetic variation in composite and parental populations: expectations for levels of dominance and gene frequency.

Usefulness of multibreed composites depends on: 1) adaptation of the average breed effects combined, 2) amount of initial heterosis retained, and 3) the rate of further improvement from selection. Potential improvement could be increased, relative to parental stocks, by retained heterosis in reproductive rate and by any increase in additive genetic variation in the composite. A single-locus, two-allele model with additive plus dominant gene effects was used to clarify expected changes from varying: 1) degree of dominance (d) from none to over-dominance, 2) variance among parental lines in frequency of a dominant allele (Vq), and 3) mean gene frequency in the composite (qc). Simulation for a three-breed composite (1/2, 1/4, 1/4) showed that expected heterosis retained in the composite is proportional to d and Vq and is highest for any d and Vq when qc is intermediate. Percentage of change of the composite from parental mean in additive genetic variance (delta Vac,%) increases most with parental diversity (Vq) when d = 0 but changes with higher d from positive when q is less than .5 to negative when q is greater than .5. Therefore, the expected association of delta Vac with level of heterosis retained (delta H,%) is always smaller for higher d and changes from positive when qc less than .5 to negative for the more likely higher equilibrium values of qc. Thus, greater selection intensity (from a higher reproductive rate) in composites than in parent stocks may not be accompanied by increased genetic variability and change in response per unit of selection applied may be limited.