Collaborative Strategies in the Molecular Genetics of the Major Psychoses

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Abstract
Most authorities now agree that a genetic contribution is the best established aetiological factor in manic-depressive illness (McGuffin & Katz, 1989) and schizophrenia (Gottesman & Shields, 1982). In both cases the size of the genetic contribution is substantial, as reflected in estimates of broad heritability (the proportion of total phenotypic variance accounted for by genes) of 60–80%. However, the evidence in favour of a genetic contribution from family, twin and adoption studies tells us that we are dealing with complex conditions which do not have simple Mendelian modes of transmission and in which environmental influences almost certainly play a part. The main debate about mode of transmission has centred upon whether it is entirely polygenic (i.e. dependent upon the action of many different genes, each of small effect) or whether for each of the major psychoses there is a gene of major effect.