Recent advances in the genetics of psychiatric disorder.

The prediction of risk for mental illnesses that have a strong genetic basis has usually been carried out by employing empirically derived estimates of recurrence observed in family studies. Because systematic methods of mapping the human genome with DNA polymorphisms have been developed it is possible that prenatal prediction of certain mental illnesses may eventually become a more accurate method of risk prediction than the empirical method currently used. Recombinant DNA technology as used in genetic linkage studies can also specify a mode of genetic transmission and lead on to aetiological studies of the disorder. The latter will result from the molecular cloning and sequencing of the responsible genetic mutations. The use of recombinant DNA technology has enabled single-gene effects to be found in schizophrenia, manic depression and Alzheimer's disease. In highly selected families it is now possible to calculate the risk of developing certain mental illnesses by DNA testing before birth. Failure to find linkages in separate schizophrenia, manic depression and Alzheimer's disease family samples has also been reported, indicating that there is heterogeneity of linkage with genes at several distinct genetic loci that contribute susceptibility to very similar clinical disorders. Resolution of the degree to which there is heterogeneity of linkage and a further examination of the factors which influence the variable effects of the abnormal genes predisposing to mental illness will both be necessary before accurate risk prediction can become feasible.