Assume that four of the loci influencing yield in corn each have two alleles (A1 and A2, B1 and B2, C1 and C2, D1 and D2). For each locus, there is one allele that adds 5 units to phenotype (A1, B1, C1, D1) and one allele that adds 0 units to phenotype (A2, B2, C2, D2).
The alleles act additively within each locus (no dominance) and across loci (no epistasis). Within each locus, genetic values are +10, +5 and 0, respectively for A1A1, A1A2, A2A2, etc. genotypes.