Genetic Models for F2 ✕ F2 and BC1 ✕ BC1 Interpopulation Crosses

Abstract

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 ✕ C1]F2 and the [I2 ✕ C2]F2 or between the [I1(I1 ✕ C1)]BC1 and the [I2(I2 ✕ C2)]BC1. To compare F2 and BC1 generations as foundation populations, expectations of F2 ✕ F2 and BC1 ✕ 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 ✕ 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.