Many genetic disorders are caused by mutations in single genes (monogenic diseases), and the inheritance pattern of these diseases follows simple rules. If a mutation in both copies of the gene on both chromosome homologues is necessary to cause the disease, the inheritance pattern is recessive, and a patient is the offspring of two clinically unaffected carriers. However, if a mutation in a single homologue is sufficient, the inheritance pattern is dominant and the disease is transmitted from generation to generation. Monogenic diseases are responsible for only a small fraction of all patients with genetic diseases. Many common diseases, including cancer, heart disease, diabetes and several psychiatric diseases, are the results of a complex interaction between genetic and environmental factors. However, these diseases can have important genetic components and can therefore still be considered genetic diseases. The identification of genes involved in complex genetic diseases can be very important for the understanding and treatment of these diseases. Over the last 15 years, much progress has been made in the identification of genes responsible for monogenic diseases, but the identification of genes involved in complex diseases has been more difficult, and at this moment little is known about the genes involved in most common diseases.

Behavioural genetics is the study of the hereditary influence on behaviour, and can therefore be regarded as the intersection between behavioural sciences and genetics. As with most other fields of research it is difficult to exactly pinpoint when behavioural genetics started. In fact, one might say that the notion behavioural traits can be inherited may have appeared in human thought as early at 8000 BC, when the domestication of the dog began.

The scientific era of behavioural genetics is generally considered to start with Charles Darwin. In his famous book On the Origin of Species by Means of natural Selection, or the Preservation of favoured Races in the Struggle for Life, published in 1859 (and sold out the first day), he devoted an entire chapter on instinctive behavioural patterns. Some years later, in his book The Descent of Man and Selection in Relation to Sex, he clearly stated that the difference between the mind of a human being and the mind of an animal ‘is certainly one of degree and not of kind’. Moreover he gave considerable thought that mental powers (and insanity) are heritable aspects.

Most forms of behaviour whether normal or abnormal, show a tendency to run in families. However, these can range from symptoms of dementia and movement disorder caused by the comparatively rare autosomal dominant Huntington's disease to common everyday aspects of behaviour such as religious persuasion or career choice. Normal behaviours and most common disorders do not show simple mendelian inheritance but instead have more complex patterns of transmission involving either major genes with incomplete penetrance, multiple genes of small effect, or a combination of the two. In addition, common complex phenotypes usually involve the combination of genetic and environmental factors. Therefore once family studies have shown that a disorder or trait is familial the next stage is to perform twin studies and, if possible adoption studies to investigate whether this results from shared genes, shared environment or a combination of the two.

Nature and nurture are not separate. Instead it is important to investigate the interplay between genes and environment and how they influence on another. To an important degree, genetic effects on behavior come about because they either influence the extent to which the individual is likely to be exposed to individual differences in environmental risk or they affect how susceptible the individual is to environmental adversities.

The current inability to explain or predict which genetically predisposed individuals will finally become schizophrenic has forced us to confront our ignorance of the precise mechanism and mode of the transmission of this disorder. Zubin's proposal to view the schizophrenic as a vulnerable individual who develops a temporary episode only under certain provocations can help place the controversy in proper perspective. Vulnerability (predisposition, diathesis) to schizophrenia can be defined as the individual's characteristic treshold beyond which stressful events produce decompensation manifest in the clinically diagnosable symptom picture.

Decades of research into the etiology of schizophrenia on a phenotypic level, i.e. studies of neuroanatomy, neuropathology, neurophysiology and other areas such as immunology have yielded only fragmentary results. A contribution of genetic factors, has been consistently shown, however, beginning with E. Kraepelin's pioneering studies at the turn of the century. Evidence has accumulated from family-, twin-, and adoption studies. Identical twins have a 48% risk of developing schizophrenia if one of them is affected. In contrast, a 17 % risk is reported for nonidentical twins. These rates are similar to other complex genetic disorders such as diabetes, hypertension and asthma. Advances in the genetic analysis of complex traits as well as progress in the Human Genome Project should provide a basis for uncovering the molecular causes of schizophrenic disorders and for investigating the neuropathology of this individually and socially devastating neuropsychiatrie disorder. There is no doubt, that discovery of the genetic variation associated with the illness would help in identifying specific targets for development of more effective, targeted treatments.

The distribution of schizophrenia within populations bears upon the genetic nature of the disorder. From the World Health Organization Ten-Country Study of incidence Jablensky et al concluded that: Schizophrenic illnesses are ubiquitous, appear with similar incidence in different cultures and have clinical features that are more remarkable by their similarity across cultures than by their difference.

The WHO study included populations in Japan, India and Europe that have been separated for tens of thousands of years. Moreover illnesses with essentially the same characteristics are commonplace in the Australian aboriginal population that separated from other human populations 50,000 years ago.

Serotonin 5-hydroxytryptamine (5-HT) is an important regulator of morphogenetic activities during early central nervous system development, including cell proliferation, migration, and differentiation as well as synapto-genesis. Serotonergic raphe neurons diffusely project to a variety of brain regions (e.g. cortex, amygdala, hippocampus) and play known roles in integrating emotion, cognition, motor function as well as in food intake, sleep, pain, and sexual activity. The diversity of physiologic functions is due to the fact that 5-HT acts as a master control neurotransmitter within a highly complex system of neural communication mediated by multiple pre- and postsynaptic 5-HT receptors, thus orchestrating the activity and interaction of several other neurotransmitter systems. Since proteins involved in the regulation of central serotonergic activity (e.g. enzymes, receptors, transporter) play pivotal role in brain 5-HT homeostasis, polymorphisms in the regulatory regions of their genes resulting in variation of expression and function are likely to influence complex traits, such as temperament/personality and psychopathology.

Alzheimer disease (AD), the most common form of dementia in the elderly, is rapidly becoming a major health problem in developed countries where the number of elderly people continuously grows due to improved medical care. Consequently, the number of AD patients is increasing and thus far no effective therapies are available. Clinically the disease can be diagnosed with 90% reliability on the basis of neurological examination, neuropsychological testing and brain imaging techniques. A definite diagnosis, however, requires the post-mortem detection of senile plaques (SPs) and neurofibrillary tangles (NFTs) in the brain. The SPs are extracellular deposits mainly composed of amyloid P (Ap) surrounded by dystrophic neurites. NFT are intraneural inclusions of paired helical filaments composed of hyperphosphorylated tau.

Although age is the major risk factor for AD, population survey and family studies have provided substantial evidence that genetic factors are major contributors to the expression of AD.

The common denominator of anxiety disorders is that they share inappropriate levels of emotions and cognitions that affect rather than enable adaptive behaviours. The variety of symptoms include ‘spontaneous’ panic attacks with mental and physical symptoms, stimulus bound anxiety associated with avoidance behaviour, and almost constant ‘generalized’ anxious feelings. According to the DSM-IV criteria the anxiety disorders are classified as shown in Table I.

Traditional methods used to asses genetic effects, such as twins, adoption and family studies, have demonstrated the role genetic vulnerability factors in the etiology of major psychiatric diseases such as affective disorders and schizophrenia. It remains however impossible, using these methods, to specify the genetic variables involved and the exact mode of transmission of these diseases. New genetic approaches in psychiatry include the use of DNA markers in sophisticated strategies to examine families and populations. Genetic linkage (in families) and allelic association (in unrelated subjects) are the most frequent techniques applied searching for genes in psychiatric diseases. Advances in these methods have permitted their application to complex diseases in which the mode of genetic transmission is unknown. Affective disorders and, in particular, bipolar affective disorder (BPAD) have been examined in many molecular genetic studies which have covered a large part of the genome, specific hypotheses such as mutations have also, been studied. Most recent studies indicate that several chromosomal regions may be involved in the aetiology of affective disorders. Large multi-centre and multi-disciplinary projects are currently underway in Europe and in the US and hopefully will improve our understanding of the genetic factors involved in affective disorders. In parallel to these new developments in molecular genetics, the classical genetic epidemiology, represented by twin, adoption and family studies, have been improved, providing validated models to test the gene-environment interactions.

Several psychiatric diseases are considered to be neuro-developmental disorders. Amongst these are schizophrenia and autism, in which genetic and environmental components have been indicated. In these disorders intrinsic molecular mechanisms of brain development may be deranged due to genetic predispositions, or modified by external influences. Brain development is a delicate process of well-tuned cellular proliferation and differentiation of multipotent neural progenitor cells driven by spatiotemporal cues. One of the fundamental mechanisms is the interaction between external signals, e.g. growth factors, and internal regulators, e.g. transcription factors. An important transmitter system involved in behavioural and affective functions relevant for psychiatric disorders is the mesencephalic dopamine (DA) system. The mesencephalic DA system is organized in two anatomically and functionally different systems. DA neurons in the ventral tegmental area project to the mesolimbic system and are mostly related to control of behaviour. It has been implicated in drug addiction and affective disorders like dipolar disorder and schizophrenia. The dopamine system of the substantia nigra (nigro-striatal pathway) is implicated in movement control. Degeneration of this system, as in Parkinson's disease, or altered function in tardive dyskinesia have highlighted its importance in human disease. Recent findings in molecular neurobiology have provided the first clues to molecular mechanisms involved in developing and mature DA neurons. These may have clinical implications in novel therapeutic strategies.

An increasing number of neurodegenerative diseases have been defined at the molecular level in recent years, making it possible to determine precisely the genotype before the onset of symptoms. Pre-symptomatic testing programs are available for Huntington disease (HD), hereditary cerebral haemorrhage with amyloid-Dutch type, inherited cerebral ataxia, myotonic dystrophy, and Alzheimer disease. Although treatment options such as gene therapy have no widespread application until now, and much has to be developed, the use of predictive DNA-diagnostics has become a clinical application for a number of hereditary diseases. For psychiatric disorders such as schizophrenia or bipolar disorders, there are indications for localisation of the genetic factors, but clinical use of genetics is still far from reality. Yet, the increasing knowledge about genetics will have far-reaching influence in most fields of modern medicine and in health care provisions.

A great interindividual variability exists in biological response to drugs. This variability is partly attributable to pharmacodynamic factors (drug - receptor interactions) and partly to pharmacokinetic factors. Drugs can be eliminated from the body by renal clearance, metabolism or both. Although every tissue has some ability to metabolise xenobiotics like drugs, the liver is the principal organ of biotransformation. Major metabolising enzymes are the cytochrome-P450 mono-oxygenases, epoxide hydrolase, glucuronosyl-transferase, acetyl-transferase, sulfo-transferase and xanthine oxidase. Some of these enzymes display in a subset of subjects a ‘normal’ activity and in another subset of subjects a reduced or a greatly increased activity. This altered activity may be genetically determined and is then called genetic polymorphism. Clinically relevant metabolic differences traditionally have been defined by their genotypie expression such as ‘poor’ and ‘extensive’ metaboliser. The recent developments of powerful methods for DNA (or genomic) analysis portends a revolutionary expansion of our understanding of physiology as well as pathology. Pharmacogenetics is the study of genetic variation underlying differential response to drugs. Genotyping may become a useful tool in optimising drug treatment. Another part of the genetic research is directed towards the discovery of genetic alterations leading to diseases. Once identified, these genetic alterations can become targets for drug treatment (e.g. gene therapy). Pharmaco-genomics applies the large-scale systematic approaches of genomics to speed the discovery of drug response markers, whether they act at the level of the drug target, drug metabolism or disease pathway. Table I gives some examples of genetic alterations that are identified together with their effects. Some of these examples will be briefly discussed here.

Neuropsychiatry or Biological Psychiatry There is an urgent need to reconsider the position of psychiatry within the neurosciences because of the exploding knowledge about the relationship between brain and behaviour and the delay in implementation of new findings due to the separation of neurology and psychiatry. Biological psychiatry and psychopharmacology originate from the discovery by chance of psycho-active compounds in the early fifties and have contributed to the scientification of psychiatry. The impact of biological psychiatry for the pathophysiology of psychiatric disorders, however, is limited as a result of its biased orientation on neurotransmitters and receptors. The neuropsychiatric paradigm integrates knowledge from several domains, such as functional neuroanatomy, genetics and endocrinology and opens new vistas for the involvement of neuronal circuits in the initiation and maintenance of behavioural disturbances. In addition, novel and more specific treatment modalities may emerge.