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Genetic Models for F₂ x F and BC₁ x BC₁ Interpopulation Crosses

Lifaco Seed Corp., Box 6055, Champaign, IL 61826-6055
DeKalb-Pfizer Genetics, Rt 1, Box 162, Thomasboro, IL 61878
Dep. of Agronomy, Univ of Illinois, 1102 S. Goodwin Ave., Urbana, IL 61801
Funk Seeds Int., Box 69, Rochelle, IL 61068

^* Corresponding author.

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)]BC. To compare F₂ and BC₁ generations as foundation populations, expectations of F₂ x F₂ and BC₁ 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.

Received for publication October 6, 1988.