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Modification of Methods for Identifying Inbred Lines Useful for Improving Parents of Elite Single Crosses¹

In many hybrid breeding programs, corn (Zea mays L.) programs for example, a common practice is to cross one inbred parent (e.g., I₁) of an elite hybrid (I₁) x I₂) to a donor inbred (I_w) and attempt to isolate a new line with improved performance in crosses to I₂. for I_w to be useful as a donor of alleles for improving I₁ x I₂, I_w must contain favorable alleles at loci (class G loci) for which I₁ x I₂ is homozygous unfavorable. A previous method for identifying such lines assumed that the number of loci for which I₁, I₂, and I_w were all homozygous favorable (class A loci) was equal to the number for which all three lines were homozygous unfavorable (class H loci). In addition, genotypic values of µ, µ, and —µ instead of the more general z + 2µ, z + µ + aµ, and z were assigned to + +, + -, and - - genotypes, respectively, where µ is one-half the difference between homozygotes, a is a measure of degree of dominance, and z is the value of the - - genotype. The previous method was modified to identify donor inbreds without the assumptions that Z = -µ and that µA = µH. A new set of estimation equations is presented and the effect of relaxing the assumption of complete dominance is described. Based on the new set of equations, I₂ lines would not need to be evaluated if information on class B loci (those for which I₁ and I₂ are + + and I_w is --) is not needed. Use of the new estimators is illustrated by applying them to a set of corn data. Results from the modified method are in closer agreement with expectations based on pedigrees and experience with the lines involved than were results using the original methods.

Key Words: Zea mays L. • Corn breeding • Pedigree breeding • Quantitative genetics • Germplasm evaluation

² Professor of plant genetics, Agronomy Dep., Univ. of Illinois, 1102 S. Goodwin Ave., Urbana, IL 61801.

Received for publication August 1, 1986.

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