Genetic models for F₂ × F₂ and BC₁ × BC₁, interpopulation crosses

Broadbase germplasm may be used to improve parents (I₁ and I₂) of an elite single cross. Composites (C₁ and C₂) may be crossed to each parent, followed by reciprocal selection either between the [I₁, X C₁]F₂ and the [I₂ X C₂]F₂ or between the [I₁,(I₁ X C₁)]BC₁, and the [I₂(I₂ X C₂)]BC₁. To compare F₂ and BC₁, generations as foundation populations, expectations of F₂ X F₂ and ВС₁ X BC₁, interpopulation means, half-sib covariances (CovHS), and specific combining ability variances (VarSCA) were derived for a one-locus, two-allele model with partial and complete dominance. Whereas means are expected to be higher in the BC₁, than in the F₂ interpopulation cross, CovHS and VarSCA are expected to be two to seven and two to three times greater, respectively, in the F₂ than in the BC₁, interpopulation cross. In studies considering introgression of broadbase germplasm to improve the elite maize (Zea mays L.) single cross B73 X Mol7H, grain moisture and plant and ear height data were consistent with these expectations. But for grain yield, VarSCA estimates in the F₂ were inordinately small relative to those in the BC₁, interpopulation cross. Because VarSCA is due to nonadditive genetic effects, more complex models involving unequal effects of multiple alleles, two-locus epistasis, and modification of dominance were investigated. Grain yield data were consistent with a model in which dominance at a quantitative trait locus is expressed only in the presence of a modifier allele at a modifier locus. In this model, the modifier allele is fixed in both parents of the elite single cross but present in near-zero frequencies in the composites.