Glossary of genetic terms

This Glossary provides simple definitions of some technical terms relating to genetics and genetic testing which are used in the DISCERN Genetics Tool and Handbook. For more comprehensive listings or detailed descriptions, please refer to the Resources.

Affected: Having a disease or condition arising from a genetic mutation
Antenatal tests: Tests conducted during pregnancy to assess the health of the foetus
Anticipation: Increasing severity or earlier onset of a genetic trait, disease or condition with each generation
Autosomal disorders: Genetic diseases or conditions caused by a mutation in a single gene or DNA sequence in the automosomes (non-sex chromosomes)
Autosomes: The chromosomes responsible for non-sex traits
Biochemical techniques: Processes for examining changes in gene products such as enzymes and proteins
Carrier: An individual who has one copy of a faulty recessive gene. They will not usually be affected but they can pass the mutation on to offspring if their partner is also a carrier or if the mutation produces a sex-linked recessive disorder or condition
Chromosomal disorders: Diseases or conditions arising due to mutations in the number or structure of the chromosomes
Chromosomes: The strands of DNA containing genes found in each cell nucleus. Humans have 23 pairs of chromosomes with one chromosome from each parent in each pair
Classical inheritance - disorders: See Single gene disorders
Congenital: Present at birth (but not necessarily genetic)
Cytogenetics: The study of the genetics of the cell. Used to detect chromosomal abnormalities
Diagnostic test: A test for detecting the presence or absence of a disease or condition. Often used following a positive screening test.
DNA (Deoxyribonucleic acid): The material inside the nucleus of cells that carries genetic information
Dominant disorders: Genetic diseases or conditions caused by a mutation on one of the genes in a pair. The single mutated gene dominates. The disease or condition can therefore be inherited from one parent who has the faulty gene
Expressivity: The extent to which a genetic disease or condition is expressed in an individual. Expressivity is demonstrated by variations in the strength or nature of traits amongst individuals with the same genetic mutation
Gene: A section of DNA that determines the inheritance of particular traits. Genes are located on chromosomes in the cell nucleus
Genetic condition: A condition, disease or disorder with a genetic cause
Genetic test: A test to detect the presence, absence or alteration in a particular gene or chromosome. This can include tests of DNA and chromosomes, gene products and functions, or other markers of a genetic mutation.
Genome: All the genetic information contained in the DNA of a set of chromosomes from a cell nucleus
Genotype: The genetic make up of an individual
Germ cells: Sex cells (ova and sperm)
Germline mutation: A mutation occurring in the sex cells. The mutation is present in every cell of an affected person's body from birth throughout their lifetime. The initial mutation may be a random or sporadic event, but it may be passed to offspring and becomes hereditary from then on.
Incidence: The number of new cases of a disease or condition in a population over a certain period of time
Inherited disease or condition: A disease or condition that has been passed on through the genes of biological parents
Karyotyping: A microscopy process for examining the number and shape of chromosomes within a cell
Meiosis: A process of cell division occurring in the sex cells (germ cells) which reduces the number of chromosomes by half in preparation for reproduction. A normal embryo will get a set of 23 chromosomes from each parent. Meiosis occurs only once between generations.
Mendelian inheritance - disorders: See Single gene disorders
Mitosis: An ongoing process of cell division which enables the body to grow and repair itself. Each non-sex cell (somatic cell) divides into two new cells which each contain a duplicate set of chromosomes
Molecular biology and genetics: Submicroscopy techniques for examining changes at the level of a single gene or DNA segment
Monogenic disorders: See Single gene disorders
Multi-factorial disorders: Conditions or diseases arising from a combination of genetic and non-genetic causes, including environmental factors
Mutation: A permanent change in DNA
Neonatal tests: Tests conducted at birth to assess the health of newborn babies
Pedigree: A symbolic representation of disease transmission through families
Penetrance: The chance of being affected by a genetic trait or condition if the mutation has been inherited. Often expressed as the percentage of individuals with a certain genotype who express its phenotype.
Phenotype: A physical characteristic or feature (also known as a trait). It is determined by genotype, but is also influenced by environment and sporadic genetic mutations
Predictive test (genetic): A test used to detect the presence of a genetic mutation in individuals who are currently well but who may be at risk of developing a genetic disease or condition
Presumptive test (genetic): See Predictive test
Pre-symptomatic test (genetic): See Predictive test
Prevalence: The total number of cases of a disease or condition in a population over a certain period of time
Prognosis: The predicted course and outcome of a disease
Recessive disorders: Genetic disease or condition caused by a mutation on both genes in a pair (homozygous). The disease or condition only arises when both parents pass on a copy of the faulty gene.
Risk: The likelihood, chance, possibility, or probability of an event
Screening test (genetic): Routine genetic testing across a population or group to identify individuals at risk of a genetic disorder or condition
Sex-linked disorders: Genetic diseases or conditions caused by a mutation on the sex chromosomes. Patterns of inheritance tend to be gender-specific
Single gene disorders: Genetic diseases or conditions caused by a mutation in a single gene or DNA sequence. Also known as Mendelian, Classical and Monogenic disorders. They are usually passed from one generation to the next
Somatic cells: Non-sex cells (not ova or sperm)
Somatic or sporadic mutation: A mutation occurring in a somatic cell rather than a germ-line cell. Somatic mutations are not usually inherited - they are not present at birth but occur during a person's lifetime. Causes are not known but exposure to environmental hazards and lifestyle factors may play a role
Somatic or sporadic diseases: Mutations and diseases occurring during an individual's life with no obvious inherited cause (see somatic mutations)
Trait: A physical characteristic or feature determined by genes. (Also known as a phenotype)
X-linked: Traits, conditions and disorders determined by genes on the X sex chromosome.
Y-linked: Traits, conditions and disorders determined by genes on the Y sex chromosome.