Glossary of Genetic Terminology

Allele – The version of each gene that a biological parent passes down to their child at conception.

Base pair – two nucleotides chemically bound together on opposite complementary DNA or RNA strands.

Benign variant – Not believed to cause disease.

Carrier testing - determines if you carry a recessive pathogenic (disease-causing) allele. Carrying one means you are a ‘carrier’ and not affected by the disorder.

Cell – the smallest unit of living material. Cells are building blocks providing structure for the body, in addition to carrying out other important jobs.

Chromosomes - are the structures that contain our DNA.

De novo mutations – DNA changes that are ‘new’ that occurred first in you and are not inherited from your parents. De novo mutations can be passed on if they are present in the germline (sperm/eggs).

Diagnostic testing - is used to identify the underlying cause of your symptoms or disease.

Diploid – A zygote is ‘diploid’ meaning the cell (and all cells arising from it) have two copies of each chromosome, one from each biological parent.

DNA – Stands for Deoxyribonucleic Acid. DNA is one of two forms of nucleic acid that can be found on chromosomes within our cells. DNA is the genetic material for all cellular life forms (organisms that are made from more than one cell) and many viruses.

Dominant – A mechanism by which a genetic trait or gene mutation can be inherited. In dominant inheritance, only one allele copy (of a given gene) from one parent has a mutation.

Embryo – early development of life. A group of cells made from a zygote is often referred to as an embryo once organs and critical structures needed for life are forming. 

Epigenetics – the study of gene expression. Epigenetic changes affect the way genes are expressed but do not affect the genetic code itself. Epigenetics is often used to study variability in disease.

Epigenetic signature – the unique way in which epigenetics influences the way the genes release the information they carry.

Exons – the coding sequence in a gene. Exons are the building blocks for amino acids which make proteins.

Fluorescent In Situ Hybridization (FISH) – a type of test that has many purposes. It can be used to analyze chromosomes looking for the presence or absence of deletions or rearrangements. It can also provide researchers with a way to visualize and map the genetic maternal including specific genes or portions of genes. 

Gametes – Our reproductive cells, for instance our sex cells. In females, gametes refer to our ova or eggs, while in males, gametes refer to sperm.

Genes – A segment of DNA defined as a unit of heredity.

Gene expression – when the information stored in our genetic code is converted into instructions for making proteins or other molecules. It is a highly regulated process and acts as an ‘on/off’ switch to control when genetic instructions are used. There are two involved processes in gene expression; transcription and translation.

Genetics – The scientific study of genes and chromosomes.

Genetic Counseling – is both an individualized and family-focused process where the risks and effects of a genetic condition are determined, support and appropriate resources are given, genetic testing (when appropriate) is discussed, and counseling is provided by a qualified genetic counselor.

Genetic Counselors – Healthcare professionals with specialized and advanced training in medical genetics and counseling. They typically hold a Master’s degree in the field and are certified through a professional association. Genetic counsellors assess and counsel individuals and their family members with a genetic disorder, or a suggestive medical history for a genetic disorder. The also meet with individuals and family members who may be at an increased risk for having and passing on a genetic disorder.

Genetic testing – this type of testing analyzes parts of an individual’s DNA to find changes (variants/mutations) that cause disease or confer an elevated risk to develop disease. It is a diagnostic medical test. Genetic testing can be performed using any tissue type, including blood.

Genetic screening – this type of test is not a genetic test. Screening simply determines which individuals may have a higher risk to have a particular condition or to carry a particular gene mutation within a specific population in question. It is not a diagnostic test. Screening test results are not always accurate and need to be followed up with diagnostic testing, such as genetic testing. It is usually performed on a blood sample.

Genome – An individual’s complete set of genetic material.

Genotype – Refers to the set of specific alleles carried.

Germline – the source of DNA for all the cells in the body (fusion of genetic information in sperm and egg union).

Germline variants – DNA changes that are present in all body cells, including the sperm or eggs. Changes in germline DNA can be passed on to offspring and are often inherited from parents.

Haploid – Cells are ‘haploid’ meaning they only contain one copy of each chromosome.

Heredity – The study of measurable changes that can be pass on to future generations.

Introns – the non-coding sequence in a gene. Introns are not building blocks for amino acids with make proteins.

Karyotype – A laboratory technique designed to look at the chromosomes for structural problems such as chromosomal arrangements and aneuploidy (missing or extra chromosomes). This test is not advanced and as such as limitations as to how closely it can look abnormal chromosome numbers and structures.

Maternal – pertains to the mother, or the mother’s family.

Mutation – An alteration in the nucleotide sequence of a DNA molecule and the primary cause of diversity among all living things. Mutations can also occur in RNA (such as with viruses). Some mutations may cause disease, others may provide a beneficial advantage, and some mutations are neutral and do not have any affect. Neutral mutations contribute to the natural variation in living things.

Negative result – Normal test results for a pathogenic (disease causing) mutation previously identified in a close family member. It may also be used to describe normal results from a genetic test that can diagnose a condition based on the presence or absence of specific findings.

Next generation sequencing (NGS) – is a massive sequencing-based technology that offers ultra-high throughput, speed, and scalability to determine the order of nucleotides in the gene or specific regions of DNA or RNA.

Nucleotides – The building blocks for both DNA and RNA.

Nucleus – a specialized structure (known as an organelle) within a cell where DNA, chromosomes, and genes reside.

Offspring – your biological children (also referred to as your progeny).

Panel (multi-gene) testing – A specialized genetic test using NGS technology where a selection of genes known to be associated with the development of a particular condition or collection of clinical symptoms under investigation are analyzed at once in hopes to find the underlying pathogenic (disease causing) gene mutation.

Paternal – pertains to the father, or the father’s family.

Pathogenic variant – Disease causing.

Phenotype – The observable characteristics or physical traits based on the genotype.

Polymorphism – Neutral (benign) variants that are more common in specific populations or locations around the world. They are seen in at a frequency of 1% or greater.

Positive result – When a pathogenic (disease causing) mutation has been identified.

Prenatal testing - determines if a developing baby has a genetic condition or has inherited a pathogenic mutation previously identified in a family member.

Pre-symptomatic testing - identifies your risk for a disease that you are not symptomatic for.

Progeny - An individual’s descendants (such as their children and their children’s children, etc.)

Recessive - A mechanism by which a genetic trait or gene mutation can be inherited. In recessive inheritance, both alleles (of a given gene) from each parent have a mutation.

Reduced (or incomplete) Penetrance – Penetrance refers to the proportion of individuals with a given genotype that express the expected phenotype. Reduced penetrance can be thought of as those who do not develop features of the disorder that are expected.

(RNA) – Stands for Ribonucleic Acid. RNA is one of two forms of nucleic acid that can be found on chromosomes within our cells. RNA is actually a copy (or transcription) of DNA and it comes in various forms essential for making proteins (rRNA, mRNA, and tRNA).

Sanger Sequencing – a type of genetic test where the nucleotide sequence of a particular gene is analyzed for possible pathogenic (disease-causing) mutations.

Sex chromosomes – The last set of chromosomes. There are two types, X chromosomes and Y chromosomes.

Sex-linked – A mechanism by which a genetic trait or gene mutation can be inherited. In sex-linked inheritance alleles (of a given gene) found on either the X chromosome or Y chromosome only are inherited from a parent(s) in either a dominant or recessive fashion. Usually, the term refers more to disorders linked to the X chromosome, sometimes called X-linked.

Somatic mutations – DNA changes not present in eggs or sperm. Somatic changes only affect certain cells or tissues and cannot be passed on to offspring as they are not inherited from parents. They are due to environmental impacts. Somatic testing is often done on tumors to determine the best course of treatment. Not all somatic mutations are pathogenic (disease-causing).

Syndrome – a collection or group of physical traits or behavioural symptoms which consistently occur together. It can also be thought of as a condition characterized by a set of associated symptoms or traits.

Uninformative negative result – is a term used to describe a type of negative result from a genetic test where the cause of the condition in the family or individual has not been identified. This is not the same as a variant of uncertain significance and cannot be used when analyzing chromosomes. With an uninformative negative result, an underlying mutation cannot be ruled out. It may have been missed for various reasons. 

Variable Expressivity – This refers to the degree to which an expected phenotype is expressed based on having a particular genotype. It can be thought of as the ‘intensity’ of a particular genotype. Individuals even within the same family can show variable expressivity, meaning that some relatives will have a milder presentation that others. Variable expressivity also refers to the range of signs and symptoms that can occur in individuals with the same genetic condition.

Variant – This is the same as a mutation, however, since changes in the nucleotide sequence do not always cause disease, the term ‘variant’ is more of an accepted term to use in medicine.

Variant of uncertain significance (VUS) – When a mutation/variant has been identified, but it is not yet clear if it is disease causing or not.

Whole exome sequencing – Using NGS technology to analyze only the coding regions of the genome (the exons) for potential pathogenic (disease-causing) mutations.

Whole genome sequencing – Using NGS technology to analyze the entire genome for potential pathogenic (disease causing) mutations.

Zygote – The union between a sperm and egg create an embryo, a single cell that has two sets of chromosomes (one from each gamete) and will generate all the cells required to create the living being. The zygote divides continually creating a group of cells in which some will become the embryo and others will become the placenta.

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