Allele - any one of a number of viable DNA codings of the same gene occupying a given position on a chromosome. For example, a single gene may control hair colour, but the variations of this gene - alleles - give some people fair hair and others dark hair.

Autoimmune disease - a group of disorders that arise when the immune system inappropriately targets the body’s own tissues.

Chromosome - one of the thread-like structures found in the cell nucleus that carry the genetic information in the form of genes.

DNA - the nucleic acid that forms the material of which the chromosomes and genes of almost all living organisms are composed. DNA contains coded instructions for the transmission of genetic information from one generation to the next and for the manufacture of all the proteins that are required for the growth and development of a whole new organism.

Epidemiological study - a statistical study on human populations, which attempt to link human health effects to a specified cause.

Familial cluster - an aggregation of people with a condition or disease within the same family.

Genes - characteristics passed from parents to offspring. Genes are encoded in genetic material, and control physical development and behaviour.

Genome - a complete set of DNA. It contains all the genetic instructions to build and maintain an organism.

Familial aggregation - grouping of characteristics of a family or its members.

Haplotype - a set of closely linked alleles inherited as a unit.

HLA - Human Leukocyte Antigens. Proteins found on the surface of cells which allow the immune system to distinguish healthy cells from those that need to be removed. Also referred to as major histocompatibility complex (MHC).

Human Genome Project - a map to identify all the genes present in the human genome.

Immune cells - cells which help protect the body against infection.

Immunogenetic - study of the interrelation between immunity to disease and genetic makeup.

Immunoglobulin - one of a group of proteins found in blood plasma that act as antibodies.

Incidence (of a disease) - the number of new cases of disease occurring in a population during a defined time interval. The number is useful to epidemiologists because it is a measure of the risk of disease.

Longitudinal study - a research study that involves observations of the same subjects over long periods of time, often many decades.

Phenotype - characteristics of an organism determined by the interaction between its genotype and environmental factors.

Polymorphism - having multiple alleles of a gene within a population.

Spatio-temproal cluster - a high number of people with a condition or disease within a defined timeframe or defined geographic area.

Susceptibility - the likelihood of an individual to develop a disease or condition.