The model for the GAW15 Problem 3 genetic simulation. Genetic loci are represented as ovals, normally-distributed polygenic/environmental variables are represented as circles (G, additive polygenic; C, common family environment; E, non-shared environment) and observed variables are represented as rectangles. The RA hazard is a continuous variable that is dichotomized into affected/unaffected before it is observed, and severity is polytomized into five levels before it is observed. Arrows indicate where effects of variables are manifested. For example, HLA-DRB1 affects both anti-CCP levels and RA hazard, but the strength of its effect on anti-CCP is controlled by Locus E genotype and the effect of HLA-DRB1 on RA hazard is controlled by Locus A genotype. The incorrect cM locations for Loci D and H are given, see the note on Table 1.