GLOSSARY Basic glossary on genetic epidemiology

This is the second of a series of three glossaries on genetic concepts used in epidemiological research that the journal is publishing with the objective of helping the reader “walk ” around the journal. The first glossary, on basic molecular geneticterms,1 provided the basis to understand theconcepts presented here. In this section, we refer to the most basic and commonly used genetic terms that epidemiolo-gists interested in this kind of research need to know. The concepts defined here are the pillars on which genetic epidemiology builds up its method-ology. Again, the list is not exhaustive and an attempt has been made to provide you with concise definitions. Hence, we encourage the interested reader to further “explore ” the classic bibliogra-phy on genetics and genetic epidemiology2–5 to complete their knowledge on this topic. CANDIDATE GENE A known gene suspected to be associated with the disease of interest on the basis of the biological function of its protein. COMPLEX TRAIT (POLYGENIC TRAIT, MULTIFACTORIAL TRAIT) Any phenotype that results from the effect of mul-tiple genes at two or more loci, with possible envi-ronmental influences too. Examples are: obesity, hypertension, hypercholesterolaemia, skin pig-mentation, cancer, etc. Discontinuous trait Trait that is either present or absent, such as birth defects and common behavioural disorders. The threshold model is used to explain discontinuous traits: a protein level has a continuous distribu-tion but the phenotype does not appear until a cer-tain threshold is reached. Continuous trait Measurable trait that is always present and that follows a normal distribution in the population. For example: height, weight, and blood pressure. CO-SEGREGATION The tendency of two traits to be jointly inherited