Having evaluated the standard variance component models and estimated a heritable component for each trait, we proceeded to evaluate the evidence for heterogeneity in genetic and environmental sources of variance in males and females. We considered a series of models with heterogeneity in variance components (heterogeneity in environmental variance only, heterogeneity in genetic variance only, heterogeneity in both variance components, and a model in which the genetic and environmental variances differed between males and females by a constant factor). We also considered models that included an X-linked or mitochondrial variance component (because those types of non-autosomal inheritance also induce sex-dependent differences in covariances between relatives). We selected the best-fitting model for each trait using the Bayesian information criteria (BIC) criterion (Materials and Methods). Four traits (cholesterol, LDL, G6PD, and erythrocyte sedimentation rate [ESR]) appeared to be influenced by a substantial X-linked component. In females, we estimate that X-linked genes account for 7.5% of the variance in normalized cholesterol values, 9.5% of the variance in normalized LDL levels, 22.8% of the variance in normalized G6PD levels, and 6.2% of the variance
