This is the last of four articles discussing the broader implications of advances in genetics

The increasing rate of discovery of genes related to disease and the development of tests for them has fostered the idea that healthy people will be able to prevent future disease by undergoing genetic testing. The genetics of many diseases are such that tests have only a limited ability to predict the clinical outcome accurately. Nevertheless, the benefits of some predictive genetic tests can be substantial, such as screening newborns for phenylketonuria and sickle cell anemia and testing of older children at risk of familial medullary thyroid carcinoma and multiple endocrine neoplasia type 2a.1 The fact remains that relatively few interventions have yet been devised to improve the outcome of most mendelian disorders.2 When no treatments are available, genetic testing can be used to avoid the conception or birth of affected offspring. Carrier screening in Sardinia, Cyprus, and elsewhere in the Mediterranean has led to an appreciable reduction in the birth of infants with thalassaemia through the use of prenatal diagnosis in couples at high risk and their selective termination of affected fetuses.3

Genetic testing (box), including prenatal testing, also carries the risks of inducing psychological sequelae4 and of making individuals vulnerable to discrimination and diminishing their privacy. Additional benefits of testing in refining clinical diagnosis and tailoring treatent may be in the offing,5 but these remain to be shown.