Quantitative genetic model
Figure 2 and Table 3 presents the correlation between asthma and FENO separated into genetic and environmental sources. These were factors uniquely related to allergen-specific IgE level, factors unique to eosinophils, factors shared by allergen-specific IgE level and eosinophils, and factors shared between asthma and FENO, as estimated in the (best-fitting) AE model (Online Supplemental method outlines how the separation into unique and shared sources has been achieved).
In the best-fitting AE model, a significant phenotypic correlation, rph, was noted between asthma and FENO(rph = 0.19; Table 3). The part of the phenotypic correlation, which can be attributable to additive genetic effects, rph-a, due to allergen-specific IgE level was statistically significant (rph-a = 0.10) and accounted for half (54%) of the correlation between asthma and FENO. All other estimates were non-significant.
Other quantitative genetic models (ACE, ADE and CE), the path coefficients from the AE model, as well as the heritability estimates, are shown in Online Data Supplement, ETable 3, ETable 4A-B.
Twin correlations and results from the quantitative genetic models obtained when the categorical variable IgE positive was used instead of allergen-specific IgE level can be found in the Online Supplemental material, ETables 5–6. Overall, the results using the categorical IgE positive were very similar to the results obtained from the continuous allergen-specific IgE level.