CASE–CONTROL STUDY An epidemiological study design in which cases with a defined condition and controls without this condition are sampled from the same population. Risk-factor information is compared between the two groups to investigate the potential role of these in the aetiology of the condition.

COMPLEX TRAIT A measured phenotype, such as disease status or a quantitative character, which is influenced by many environmental and genetic factors, and potentially by interactions in and between them.

EFFECT SIZE The extent to which a factor influences the risk of the condition under study, rather than simply an indication of whether a factor is significantly related to the condition.

EPIDEMIOLOGY A discipline that seeks to explain the extent to which factors to which people are exposed (environmental or genetic) influence their risk of disease, by means of population-based investigations. Epidemiological studies are designed to minimize bias in obtaining results for the population under study.

GENETIC DRIFT The random fluctuation in population allele frequencies as genes are transmitted from one generation to the next.

GENOMIC CONTROL Statistical/genetic procedure in which the apparent association between a particular polymorphism and a certain condition is adjusted for population stratification in the study sample using a set of randomly selected, unlinked markers. Population stratification can occur if the study sample consists of two or more sub-populations with distinct differences in allele frequencies.

HAPLOTYPE PHASE The arrangement of alleles at two loci on homologous chromosomes. For example, in a diploid individual with genotype Mm at a marker locus and genotype Aa at the other locus, possible linkage phases are MA/ma and Ma/mA, for which '/' separates the two homologous chromosomes.

HAPLOTYPE TAGGING The concept that most of the haplotype structure (allele combination) in a particular chromosomal region can be captured by genotyping a smaller number of markers than all of those that constitute the haplotypes. The crucial markers to type would be those that distinguish one haplotype from another.

INCOMPLETE PENETRANCE A situation in which the probability of having the disease, given that one has the disease mutation(s), is less than 1.0.

LINKAGE DISEQUILIBRIUM Two loci that are in linkage disequilibrium are inherited together more often than would be expected by chance.

MICROSATELLITE A class of repetitive DNA sequences that are made up of tandemly organized repeats that are 2–8 nucleotides in length. They can be highly polymorphic and are frequently used as molecular markers in population genetics studies.

MINOR ALLELE FREQUENCY The lowest allele frequency at a locus that is observed in a particular population. For single nucleotide polymorphisms, this is simply the lesser of the two allele frequencies.

MULTIFACTORIAL DISEASE A disease that is influenced by many environmental and genetic susceptibility factors (see also complex trait).

POWER The probability of a study to obtain a significant result if this result is true in the underlying population from which the study subjects were sampled.

PROSPECTIVE COHORT STUDY Longitudinal analysis in which individuals selected for certain exposure characteristics are followed up over time to assess who develops a certain outcome (often disease).

TYPE I ERROR The probability of rejecting the null hypothesis when it is true. For genetic association studies, type I errors reflect false-positive findings of associations between allele/genotype and disease.