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Correlated Responses and Responses to Index Selection Involving Yield and Ear Height of Maize¹

Replicated selection studies were undertaken to evaluate the predictability and repeatability of correlated responses and responses to selection indexes. In each of six samples of the variety ‘Jarvis’ (Zea mays L.), first generation families were selected for five criteria; yield, ear height, and three indexes based on both yield and ear height. In three additional samples, families were selected for yield alone. Each sample was composed of 300 full-sib families evaluated in a different location-year environment.

Evaluation of responses to divergent selection showed that responses of both traits were generally symmetrical. Large average deviations from predictions were observed for correlated responses and responses to index selection. Variation among replicates for correlated responses was no greater on the average, than variation for direct responses. Responses of component traits to index selection were more variable in general, than responses to singletrait selection. Therefore, index selection responses tended to be less repeatable than responses to single trait selection. Although correlated responses to single trait selection deviated considerably from expectation, they appeared to be as repeatable as the direct responses, which agreed satisfactorily with expectation. Further investigation of the relationship between the two traits suggested a nonlinear association. Failure to account for this nonlinearity in predictions and indexes may have contributed to the discrepancies between observed and expected results.

Key Words: Prediction of selection responses • Genetic corrrelation • Repeatability of selection response

² Professor; research geneticist, ARS, USDA and associate professor; and professor, respectively, Dept, of Genet., N. C. State U.

Received for publication June 29, 1974.