Statistical Analysis
Observed and expected genotype frequencies were compared using a chi-square test to determine the presence of Hardy-Weinberg equilibrium in the purebred populations. Prior to the analyses, WW, AGE, BF, and LMA measurements were adjusted to a standardized age (21 days) or weight (240 lb) using guidelines provided by the National Swine Improvement Federation in order to reduce the number of covariates in the model. Data were analyzed using the GLM procedure of SAS v. 6.12 (1990). All models included the fixed effects of ESR genotype (3 classes), breed composite (4 classes), sex (2 classes), and season (5 classes) and interactions including ESR genotype x breed and ESR genotype x sex. Season described the month in which the individual was born and was classified across years. For TN, IBW, and WW data, parity of the dam was included in the model as a fixed effect. Covariates that were used included number of fully formed fetuses and gestation length for IBW; birth weight and number weaned (including transfers) for WW; and on-test weight for ADG. Linear contrasts were performed between levels of ESR genotype in order to test for the presence of dominance gene action. Least-squares means of ESR genotype were regressed on the number of copies of an allele in order to test for additive gene action.