Epistasis in Maize (zea Mays

ISHER (1918) provided a framework for the characterization of genotypic variance whereby he partitioned it into three components: ( 1 ) a portion associated with the average effects of genes, (2) a portion associated with dominance effects (allelic interactions) of genes, and (3) a portion associated with epistatic effects (nonallelic interactions) of genes. A logical method for the further subdivision of the epistatic variance was developed by COCKERHAM (1954) and KEMPTHORNE ( 1954, 1955). MATHER (1 949) introduced tests of generation means for epistasis. Procedures for the separation of epistatic from additive and dominance variation in generation means were developed by HAYMAN (1958, 1960). Numerous investigations of quantitatively inherited characters have been conducted for the estimation of genetic effects and variances. In many of these investigations, the researcher has included only additive and dominance effects in the genetic model and has assumed that epistatic effects are negligible. Although this assumption may be reasonable far some populations, it is difficult to envision a genetic system associated with a quantitative trait in which nonallelic interactions are absent. Evidence from several recent studies indicates that epistatic effects are statistically significant for quantitative traits in many species. HOWever, the degree of importance attributed to epistatic effects appears to be predicated, at least in part, on the type of experimental approach and model used. (In the following review, the term significant will be used to denote statistical significance.) Evidence for significant epistatic effects has been reported in the inheritance of quantitative traits for several autogamous plant species. In a study of genetic effects at two independent loci, LEE, COCKERHAM and SMITH (1968) reported a significant amount of epistatic variance for gossypol level in two varieties of Gossypium hirsutum L. HANSON and WEBER (1961 ) , in an F2 soybean population, reported significant additive-by-additive epistatic variance for percent oil. BRIM and COCKERHAM ( 1961 ) found evidence for significant additive-by-additive epistatic variances for maturity, height, percent protein, and percent oil in soybean populations.