Basic Glossary on Genetic Epidemiology N Malats, F Calafell
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480 J Epidemiol Community Health: first published as 10.1136/jech.57.7.480 on 1 July 2003. Downloaded from GLOSSARY Basic glossary on genetic epidemiology N Malats, F Calafell ............................................................................................................................. J Epidemiol Community Health 2003;57:480–482 This is the second of a series of three glossaries on pathways and several enzymes (some of them genetic concepts used in epidemiological research that polymorphic) involved in each way. Abnormalities in these processes have been implicated in cancer the journal is publishing with the objective of helping the and aging. reader “walk” around the journal. .......................................................................... EPISTASIS Gene interaction and, particularly, interaction between different alleles at different genes. he first glossary, on basic molecular genetic Epistasis can occur at the same step or at different terms,1 provided the basis to understand the stages of the same biochemical pathway. Tconcepts presented here. In this section, we refer to the most basic and commonly used genetic terms that epidemiolo- FAMILIAL AGGREGATION gists interested in this kind of research need to A tendency of a disease to cluster in families, know. The concepts defined here are the pillars on which is generally taken as evidence for the exist- which genetic epidemiology builds up its method- ence of a genetic aetiological mechanism, or envi- ology. ronmental factors common to family members, or Again, the list is not exhaustive and an attempt a combination of both. Ascertainment bias should has been made to provide you with concise be seriously considered. definitions. Hence, we encourage the interested reader to further “explore” the classic bibliogra- FAMILIAL ANTICIPATION phy on genetics and genetic epidemiology2–5 to The younger age of appearance of a late onset trait complete their knowledge on this topic. in successive generations. A typical effect in repeat expansions, in which severity of the disease is pro- CANDIDATE GENE portional to repeat length, which tends to grow in each transmission. A known gene suspected to be associated with the disease of interest on the basis of the biological function of its protein. FOUNDER EFFECT A change in the population allele frequency that occurs when a subpopulation is established by a http://jech.bmj.com/ COMPLEX TRAIT (POLYGENIC TRAIT, small number of individuals. The change occurs MULTIFACTORIAL TRAIT) only by chance because the members of the new Any phenotype that results from the effect of mul- population are a random subsample that may tiple genes at two or more loci, with possible envi- deviate from the overall allele frequencies. Such ronmental influences too. Examples are: obesity, changes are stronger in smaller founder popula- hypertension, hypercholesterolaemia, skin pig- tions, given the higher sampling variance. mentation, cancer, etc. GENETIC HETEROGENEITY on September 23, 2021 by guest. Protected copyright. Discontinuous trait Trait that is either present or absent, such as birth Process by which a phenotype can be caused by defects and common behavioural disorders. The different loci. A complex example is epilepsy, threshold model is used to explain discontinuous which may be attributable to different causes in traits: a protein level has a continuous distribu- different individuals: single gene disorders, multi- tion but the phenotype does not appear until a cer- factorial inheritance, chromosomal disorders, or tain threshold is reached. even brain injuries. The last case is a phenocopy. Continuous trait GENOTYPE Measurable trait that is always present and that The genetic constitution of an organism, which is follows a normal distribution in the population. modulated by the environment before being For example: height, weight, and blood pressure. expressed as a phenotype. See end of article for authors’ affiliations ....................... CO-SEGREGATION HAPLOTYPE Correspondence to: The tendency of two traits to be jointly inherited. Set of allelic states found at neighbouring loci in Dr N Malats, Institut a chromosome, as inherited from a parent. Haplo- Municipal d’Investigació types can be broken down by recombination.A Mèdica, Carrer del Dr DNA REPAIR haplotype shared among unrelated individuals Aiguader 80, E-08003, Major cell defence system against DNA damage affected with a genetic disease may indicate that Barcelona, Spain; [email protected] produced by environmental and endogenous a gene causing the disease maps to that genomic ....................... compounds. There are several different repair region. www.jech.com Genetic epidemiology 481 J Epidemiol Community Health: first published as 10.1136/jech.57.7.480 on 1 July 2003. Downloaded from HARDY-WEINBERG EQUILIBRIUM LINKAGE State in which the allele and genotype frequencies do not change The phenomenon whereby phenotypes and alleles at one or more from one generation to the next in a population. It requires marker alleles tend to be inherited together more often than random mating and the absence of selection, mutation, expected. Linkage usually means that a gene contributing migration, and genetic drift. In Hardy-Weinberg equilibrium, partially or completely to the phenotype (a genetic disease, for allele and genotype frequencies are related through the instance) maps in the vicinity of the markers. Hardy-Weinberg law: for a locus with two alleles P,Q at fre- quencies p and q respectively, homozygotes for P are found at frequency p2, homozygotes for Q have a frequency q2, and het- MARKER erozygotes are found at a frequency 2pq. Although conditions Any neutral polymorphism used in linkage or association analysis. for Hardy-Weinberg equilibrium are seldom strictly met, genotype frequencies are usually consistent with the Hardy- MEIOSIS Weinberg law. Some useful software packages to test whether Process whereby four haploid germ cells (gametes) are a set of genotypic frequencies conforms to Hardy-Weinberg produced from a diploid parent cell for sexual reproduction. are Arlequin (http://anthropologie.unige.ch/arlequin/) and During meiosis crossovers occur between homologous chromo- Genepop (http://wbiomed.curtin.edu.au/genepop/), among somes so that each chromosome found in the gamete consists others. of a patchwork of material from both members of the pair. INHERITANCE Pattern followed by the transmission from generation to gen- METABOLISM (OF EXOGENOUS AND eration of a given phenotype, usually a disease. ENDOGENOUS CHEMICAL COMPOUNDS) Cellular system of enzymes (most of them polymorphic) that Complex inheritance (non-Mendelian inheritance) activates and deactivates chemical compounds through Variability in phenotype expression that is attributed both to the chemical radicals. Metabolic enzymes are classified in two inheritance of combinations of alleles at multiple loci and to groups according their most important function, activating or environment exposures. deactivating, and in several families. Multifactorial inheritance Complex inheritance in which multiple genes are involved MITOSIS jointly with environmental influences. Asexual reproduction of a somatic cell in which the two daughter cells each have a genetic makeup that is identical to Polygenic inheritance that of the parent cell. Complex inheritance in which multiple genes but no environ- mental factors are involved. PENETRANCE Mendelian inheritance The likelihood, or probability, that a particular genotype will Simple pattern of inheritance that follows the rules set out by be expressed in the phenotype. A penetrance of 100% means Mendel. Mendelian traits are determined by just one genetic that the associated phenotype always occurs when the corre- locus, with complete penetrance and no phenocopies. Mendelian sponding genotype is present. Similarly, if only 30% of those inheritance can be dominant, recessive,orsex linked. carrying a particular allele (such as a disease-causing mutation) exhibit a phenotype (the disease), the penetrance is http://jech.bmj.com/ Dominant inheritance 30%. Type of inheritance in which one copy of an abnormal gene is sufficient to cause disease (for example, Huntington’s PHENOCOPY disease). If penetrance is complete, the abnormal gene is inher- An environmentally caused phenotype that mimics a genetic ited from a parent who also has the disorder and every trait. For example, epilepsy can be caused by mutations in sin- generation in the family has members with the disorder. gle genes (with genetic heterogeneity), and, among other causes, by brain injury, which produces a phenocopy of genetic Recessive inheritance on September 23, 2021 by guest. Protected copyright. epilepsy. Type of inheritance in which two abnormal copies of the gene must be present for the individual to be affected (for example, cystic fibrosis). Each parent contributes one abnormal copy of PHENOTYPE the gene to the child who has the disorder. Heterozygous Expressed traits or characteristics of an organism, regardless individuals (such as the parents of the affected) are called of whether or to what extent the traits are the result of geno- carriers of the disorder because they have one normal and one type or environment, or of the interaction of both. For exam- abnormal copy of the gene, but they do not show symptoms of ple, hair colour, weight, or the presence or absence of a disease. the disorder. Sex linked inheritance (GENETIC) POLYMORPHISM Type of inheritance followed by the traits caused by genes Genome