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Cumulative Gene Effects and the Nature of Heterosis in Maize Crosses Involving Genetically Diverse Races¹

Five open-pollinated varieties each representative of a different racial group and 35 other populations developed from them (the five varieties selfed, 10 F₁ crosses, and the random mated and selfed progenies of the F₁ crosses), were grown in three environments in Mexico. Data were taken on grain yield, days to flower, plant height and number of ears per 10 plants. The phenotypic means of the 40 populations for each trait were analysed separately by analysis of variance technique, and the variance among populations was subdivided according to the model and analysis I outlined by Gardner and Eberhart (1966).

In the case of yield, the greatest contributing factor to genetic variation was intravariety dominance effects (52.4% of the total). Additive gene effects contributed most to variation among means in days to flower (87.9%), plant height (79.3%) and ear number (60.9%). Average heterosis in the crosses explained 5.1% of the total variation in yield means whereas varietal and specific heterosis effects accounted for only a minor portion of the variation and were nonsignificant statistically. In the other traits, "the variation accounted for by heterosis effects was, in all cases, nonsignificant.

Estimates of the gene effects were used to compute predicted means of the entries in the experiment, the advanced generations of crosses of three and four races and more precise estimates of heterosis. The highest yielding predicted crosses in advanced generations are suggested as basic genetic material to be used in a mass selection program prior to initiating any breeding procedure involving inbreeding.

The data obtained are considered to fit the genetic model used. No evidence was found to indicate that epistasis contributes to heterosis in the traits studied even though genetically diverse varieties of maize were crossed. The possible presence of undetectable types of epistasis and the stability of the estimated gene effects in different environments are discussed.

Key Words: genetic diversity • epistasis • inter-racial crosses

² Former graduate student and Rockefeller Fellow, University of Nebraska (now Corn Breeder, CIMMYT, Mexico); Professor of Agronomy, University of Nebraska; and former C. Petrus Peterson Professor of Agronomny, University of Nebraska (now Director del Progxama de Mejoramiento de Malz, CIMMYT, Mexico) respectively.

Received for publication August 9, 1967.

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