1 Common Effects Sex-limitation Model

The common effects sex-limitation model is simply one in which the sex-specific pathways in Figures 9.1 ($a'_{m}$ or $d'_{m}$) are fixed to zero or the additve or dominant genetic correlation between males and females is fixed to .50. As a result, only the genetic effects which are common to both males and females account for phenotype variance and covariance. Although the genes may be the same, the magnitude of their effect is still allowed to differ across the sexes. This restricted model may be compared to the general sex-limitation model using a $\chi^{2}$ difference test with a single degree of freedom. Information to discern between the general sex-limitation model and the common effects model comes from the covariance of DZ opposite-sex twin pairs. Specifically, if this covariance is significantly less than that predicted from genetic effects which are common to both sexes (i.e., less than $[(a_{m} \times a_{f}) + (d_{m} \times d_{f})]$), then there is evidence for sex-specific effects. Otherwise, the restricted model without these effects should not fit significantly worse than the general model. Mere inspection of the correlations from DZ like-sex and opposite-sex pairs may alert one to the fact that sex-specific effects are playing a role in trait variation, if it is found that the opposite sex-correlation is markedly less than the like-sex DZ correlations.