Fig. 1.

A SCC model depicting a disease caused by genotypes at two loci, G and H, acting independently (sufficient causes I and II) and together (sufficient cause III). Sufficient cause IV represents the causes of disease that do not involve genotypes at the G, and H loci. G₁ denotes the causal (i.e. predisposing) genotype at locus G, and H₁ denotes the causal genotype at locus H. U₁, U₂, and U₃ are the ‘causal partners’ required for the respective predisposing genotypes to cause disease (i.e. determinants of penetrance of G₁ and H₁).