The pattern of changes of the genetic covariance between two characters on selection was examined in an effort to explain the asymmetry of correlated responses in two traits, or of the same trait in two environments, frequently observed in experimental results.The algebraic conclusions were further examined by model selection experiments using a computer. The computer was programmed to calculate the change in gene frequency from generation to generation and to calculate from it the expected changes in genetic variances and covariance as selection proceeded. This procedure was carried out with several models of gene effects and gene frequencies.Asymmetry of the genetic covariance, and consequently of the correlated responses, resulted when the relative change in gene frequency at the loci contributing positively and negatively to the covariance depended on the trait selected. The conditions necessary for the development of asymmetry were examined and the results suggest that any symmetry found in an experiment is perhaps more surprising than asymmetry. Probably the most frequent contribution to asymmetry in practice will be from loci contributing negatively to the covariance and having frequencies other than 0·5.Accurate prediction of correlated response over many generations is therefore not possible without prior knowledge of the composition of the genetic covariance, as well as its magnitude. The validity of existing theory for the prediction of correlated responses is likely to be much poorer than for the prediction of direct responses. Predictions would then have to be based on the genetic parameters estimated in each generation.

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Parameters of the logistic function of growth, fit to individual body weight curves of two randombred control populations of each sex of chickens from hatching to 45 weeks of age, were evaluated. Growth-rate constant and age at the infection point in the curve were estimated by the method of sample quantiles from individual weekly body weights of 225 males and 281 females of the Rhode Island Red (RIR) line, and 164 males and 239 females of the White Leghorn (WL) line. Heritability estimates, based on correlation among full-sibs, of growth rate constant were 0.18 +/- 0.32 in males and 0.29 +/- 0.29 in females of the RIR line, and 0.41 +/- 0.40 in males and 0.46 +/- 0.32 in females of the WL line. Estimates of heritability of age at the inflection point were 0.36 +/- 0.44 in males and 0.42 +/- 0.32 in females of the RIR line, and 0.46 +/- 0.41 in males and 0.50 +/- 0.28 in females of the WL line. Observed variation for each trait probably does not provide evidence for heritable differences. No genetic correlations were evident among growth-rate constant, age at the point of inflection, and initial or maximum weight. According to these results, it does not appear that selection for growth-rate constant or age at the point of inflection will change the shape of the growth curve of these populations genetically. Moreover, correlated genetic change in initial or maximum weight would not be expected.