Observed correlations
We presented correlations from the “Symmetric” model described above.
The phenotypic correlations, rph, were based on the
within-twin/cross-trait correlations. Intra-class correlations (ICC) are
the correlations between the same variable measured in one twin and in
his/her co-twin (i.e., cross-twin/within-trait). If the absolute value
of ICC is larger in MZ twins than in DZ twins, this indicates that genes
are involved in the association. The cross-twin/cross-trait (CTCT)
correlations represents the correlations between one variable in one
twin and another in the co-twin. If the absolute value of the CTCT is
larger in MZ twins than in DZ twins, this implies that genes influencing
both traits (at least partly) overlap. Pearson correlations were
calculated for the associations between continuous traits, while
tetrachoric for binary (asthma) and biserial correlations were
calculated for both binary and continuous traits.