Old and new ideas about genes and behaviour.

Genetic factors have long been recognized as contributors to variantion in behaviour both within the normal span and as mental diseases. The first attempts to make behaviour the subject of scientific genetic studies used likeness between twins and other relatives to confirm heredity. Later heritability has been used as a quantitative estimate of the genetic part of the variance. Attempts to localize genetic factors became possible when associations between phenotypic aberrations and karyotype were observed. Inborn errors of metabolism further confirmed that specific metabolic deficits could influence behaviour. Many kinds of common mental deficiencies such as senile dementia have a heterogeneous background, and so have normal variations in talents and personality. The mapping of the human genome, the availability of an unlimited number of genetic markers and efficient statistical tools promised ample discoveries of genes behind the variation. The expectations have not been fulfilled and more subtle influences on gene expression have to be assumed. Topics that are taken up include genomic imprinting, brain activation patterns, the importance of neurotransmitter regulation and non-additive interactions between genes and environment.