= Solution
For a binary disease <phenotype>, put $p_i(g)=P(X_i=\text{affected}\mid G_i=g)$. The <penetrance> function depends on the inheritance model, the disease <alleles> and <genotype>, and potentially age, sex, environment and other genetic modifiers. Phenocopies allow disease without the putative disease <genotype>, while incomplete <penetrance> allows a <genetic carrier> of that <genotype> to remain unaffected. Thus the <phenotype> factor is $p_i(g)$ for an affected individual and $1-p_i(g)$ for an unaffected one.
Under a fully penetrant model with no phenocopies the <genotype> determines disease status, so \b[the factor is 1 for a compatible <genotype> and 0 for an incompatible <genotype>]. For example, in fully penetrant <recessive inheritance>, $aa$ is affected and $AA,Aa$ are unaffected. Missing <phenotype> observations contribute the constant factor 1 rather than excluding <genotypes>.
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