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Characteristics of the Elite Population Selected on Ratio Criterion: II. Traits with Unequal Genetic Variances

USDA-ARS, Mississippi State, MS 39762

^* Corresponding author.

Individuals of a population can be selected simultaneously for two positively valued metric traits X and Y based on their ratio (X/Y) the ratio criterion (RC). The ratio calculation is simple and sometimes has an intrinsic biological or economic significance. In this simulation study, the traits X and Y have normal distributions, genetic variances summing to 2, 3, or 5, and ratios of genetic variances of numerator to denominator traits that are 4:1, 2:1, 1:1, 1:2, and 1:4. As in an earlier study, responses to RC selection were simulated for genetic correlations between traits X and Y of – 0.75, –0.50, –0.25, 0.0, 0.25, 0.50, and 0.75. For genetic correlations of –0.75, –0.50, –0.25, 0.0, and 0.25, the RC selection caused one trait to increase in value while the second decreased for each ratio of variances. for highly positive genetic correlations of 0.50 and 0.75, the ratios of genetic variances had differential qualitative effects on the responses. Depending on the ratios and whether selection was for an increase or decrease in the ratio, the means of traits X and Y may both increase or both decrease or one may change while the second is unchanged. With RC selection, the effects on component traits X and Y may be undesirable. It is suggested that in the presence of any positive genetic correlation, RC selection should be executed with care.

Received for publication June 1, 1995.