Chunk #119 — 5. Implications for understanding gene-brain-behavior relationships in health and disease — 5.1. Intermediate phenotypes, or “endophenotypes”
Of course, in reality, very few candidate endophenotypes would meet all these criteria. A question arises, what of the criteria listed above are most imperative in order for a measure to be an intermediate phenotype. Since, by definition, it is imperative that a measure or construct mediating genetic influences on behavior is genetically determined, heritability appears to be the main criterion. Furthermore, a demonstration of heritability automatically serves as a proof of reliability, since test-retest reliability theoretically serves as the upper limit for heritability. The latter pertains to the stable, trait-like variance; consequently, a non-stable measure cannot be heritable. It is important to note that the reverse is not true: high test-retest stability does not necessarily imply heritability, because stable individual differences can also be shaped by environmental factors. The intermediate phenotype approach is illustrated below using the example of the oddball P3 ERP component as a marker of risk for alcoholism and associated externalizing behaviors syndrome.
