We also postulate that within locus heterogeneity is likely, though not yet clearly documented in addiction, to our knowledge. Many common Mendelian disorders and rarer Mendelian phenocopies of common disorders display substantial heterogeneity within their pathogenic loci. A number of variants in the same CFTR gene produce cystic fibrosis disorders [92]. α synuclein missense variants and copy number variants can each provide phenocopies of idiopathic Parkinson’s disease [93]. Evidence for within-locus heterogeneity in complex disorders is just beginning to be accrued; such evidence now includes data from neurexin gene family variants in autism [94–97].
