The world of genetic medicine is full of complicated terms and phrases.

They may trip easily off the experts’ tongues but chances are they leave you feeling baffled. 
As you turn up for appointments at hospitals and clinics, you may feel more in control if you’re able to understand some of these commonly-used terms.

That’s why we have compiled this jargon-buster – to help you make sense of hard-to-understand or commonly-used phrases.

We’ve made a start on the list but it’s far from comprehensive and we are adding to it all the time (please let us know, too, if there’s a word or phrase you think should be there and we’ll put it in, along with an explanation).

Advanced maternal age

women over the age of 34 (35 at delivery)


A genetically inherited condition in which there is a marked deficiency of pigmentation in the skin, hair and eyes. Individuals with these traits are called albino. The gene for albinism is recessive.


A person inherits two copies of any one gene: one from each parent.  The DNA sequence of the different copies may be slightly different.  Each one is called an allele. Even simple characteristics, like eye colour, are determined by at least one pair of alleles.

Alpha-fetoprotein (AFP)

A protein excreted by the foetus into the amniotic fluid, then through the placenta to the mother’s bloodstream

Amino acids

Small molecules that are the components of proteins. There are 20 different kinds of amino acids in living things. Proteins are composed of different combinations of amino acids assembled in chain-like molecules. Amino acids are primarily composed of carbon, oxygen, hydrogen and nitrogen.


Prenatal diagnosis method using cells in the amniotic fluid (the fluid surrounding the unborn baby) to determine the number and kind of chromosomes of the foetus


Cells obtained by amniocentesis


Any process by which specific DNA sequences are replicated, or copied, disproportionately greater than their representation in the parent molecules


When someone has a variant chromosome number (too many or too few), for example in Down syndrome or Turner syndrome

Artificial insemination

When sperm is placed into the female reproductive tract, or male and female gametes are mixed by other than natural means


Any chromosome that’s not involved in sex determination (sex is determined by the sex chromosomes, X and Y). Humans have 22 pairs of autosomes.

Autosomal dominant inheritance

A type of inheritance where just one faulty copy of a gene is all it takes to cause a genetic disorder.  Somebody with a condition with this type of inheritance has a 50% chance of passing it to each of his/her children.

Autosomal recessive inheritance

A type of inheritance where both copies of a gene must be faulty for a genetic disorder to occur.  One copy on its own isn’t ‘strong’ enough.  For a recessive condition to happen, someone has to inherit two copies, one from each parent. People who have only one copy of an unusual allele (these people are often called ‘carriers’) are usually healthy and have no symptoms of the condition.  


A virus whose host is a bacterium. Commonly called a ‘phage’

Base pair

Also known as ‘bp’, these are two nitrogenous bases or chemical units (adenine and thymine, or guanine and cytosine) which are held together by weak bonds. Two strands of DNA are held together in the shape of a double helix by bonds between these base pairs.

Base sequence

A partnership of organic bases found in DNA and RNA; adenine forms a base pair with thymine, and guanine forms a base pair with cytosine in a double-stranded nucleic acid molecule


It is possible to have one defective gene and be completely unaffected. In these instances, this is because the gene causing the disorder is recessive and overridden by a dominant gene. For some conditions like thalassemia, carriers can be affected but to a lesser extent. Every one of us is a carrier of around 6-8 recessive genes.


The basic structural and functional unit of life.  As humans, we are made of approximately 50 trillion cells, including hair cells, skin cells, blood cells and eye cells. They all contain the parts essential to life, including our DNA.

Chorionic villus sampling (CVS)

If you believe there is a high chance of passing a serious genetic disorder on to your unborn child, it is possible to determine this from between 10-13 weeks of pregnancy. The CVS test involves inserting a needle through the abdominal wall and taking a sample of cells from the placenta for chromosomal testing. Some women prefer not to have it and others prefer to be able to make an informed choice.


A long threadlike strand of DNA that carries a set of genes. Normally, humans have 46 chromosomes in each cell (23 from each parent). Of these, 22 are autosomes and 1 is a sex chromosome (either X or Y).


A condition or characteristic that is present from birth.

Contiguous genes

Genes physically close on a chromosome that, when acting together, express a phenotype


A situation in which people are genetically related. Cosanguinous individuals have at least one common ancestor in the preceding few generations.


The study of chromosomes.


The loss of a segment of the genetic material from a chromosome.

De novo mutation

Also called a ‘sporadic mutation’ – a “new” alteration to a gene that is seen for the first time in a family.  These new mutations are not inherited from a parent, but someone who has it can then pass it down to their children and so on. Conditions caused by sporadic mutations are due to an alteration to a gene that occurs out of the blue in either the egg or sperm near the time of conception, or just afterwards.


A cell in which all the chromosomes occur in pairs.  All cells in the human body are diploid, and carry 23 pairs of chromosomes for a total number of 46, with the exception of the mature sperm and egg cells, which are haploid.


Our bodies have a good level of self-regulation to protect the processes involved with our fragile DNA structures. However, on the rare occasion a mutation occurs, a whole spectrum of physical, behavioural or mental functions can be impacted. Disorders can be passed down even if parents don’t show any signs of having a disorder.


Found within the nucleus of a cell, this is the primary carrier of genetic information. It is famous for its double helix structure (which looks a bit like a twisted ladder) and it holds the genetic instructions to our development and life as human beings. Humans share 99.9 per cent of the same DNA as each other, with just 0.1 per cent giving us our unique characteristics. DNA is found in almost all cells and is best seen under the microscope when condensed into chromosome form.

DNA probe

A biochemical used to identify or isolate a gene, a gene product or a protein.

Dominant allele

This is an allele which will completely hide the effects of a recessive allele.


Enzymes are essentially specialist proteins that speed up the rate of reactions inside the body. Enzymes can make all kinds of biochemical reactions possible. For example, there is an enzyme that unzips the DNA double helix so that the genetic code can be read. There are also enzymes that are made according to our genetic codes for other purposes such as digesting food. Their structures are as specific as their purpose and even a small change in the genetic instructions could lead to a completely ineffective enzyme.


The amount someone is affected by one of their genes.  For example. two children might have the same faulty gene but how much they’re affected could be completely different.


Related or pertaining to the family group

Fragile site

A site on a chromosome that is vulnerable to breakage or mutation


Sex cells, in other words a sperm or an egg.


The fundamental unit of heredity which carries the instructions for how the body grows and develops. When someone says you’ve got your mum’s eyes or your dad’s nose, it’s because your genes come from your parents. Genes refer to the specific portions of DNA that code for something in particular, like eye colour or blood type. We all have different sets of genes which make us unique to someone else.

Gene therapy

Treatment of a genetic condition by putting a new gene into the affected cell. The extra gene makes up for the gene that is not working properly.  The gene that’s inserted into the cell makes some form of protein and this allows the cell to work properly again.

Genetic counselling

Information and support provided by a specialist doctor (usually a geneticist) to parents when there are known conditions in their families or parents who are concerned about the future possibility of transmitting a genetic condition.


All the genes carried by a single gamete (sperm or egg). The genome is the DNA content of an individual, including all 44 autosomes, 2 sex chromosomes (XX or XY) and the mitochondrial DNA.


The genetic information carried by someone’s genes. We can’t see the genotype but it determines a particular characteristic, like the colour of our eyes. The expression of the genotype is called the phenotype.


A cell that has a single set of unpaired chromosomes.  Mature sperm and egg cells are haploid, each carrying only 23 single chromosomes.


Possessing two different alleles of a particular gene.


Possessing two identical alleles of a particular gene.


The expression of a gene depends on what gene it is paired with. Homozygotes are guaranteed expression of a gene because the pair they possess are identical.

Housekeeping genes

Genes present in all cells because they provide functions needed for sustenance of all cell types.

Human Genome Project

A major research project started in 1990 and finished in 2003, to map each human gene and its location on the chromosome.  Knowing where genes are located is the first step in being able to develop treatments and, eventually, cures for genetic disorders. There are more than 20,000 genes to identify so even though the project officially ended in 2003, the study continues.


This is the rate new cases for a condition occur within a given population.


The genes we inherit are random. Their expression depends on dominance within pairs of genes. Inheritance is the passing on of genes and traits from parent to child.

In vitro

Outside a living organism


The passing down of genetic information from parents to children.


How many chromosomes somebody has and any abnormalities them may have.  A karyotype is also a diagram of somebody’s chromosomes.


The precise location on a chromosome where a gene is found.


After conducting experiments cultivating his garden peas, Austrian monk Gregor Mendel built a principled framework for the rules of inheritance. Mendelian refers to his laws of heredity and remains the basis of genetics today.


A permanent alternation to a gene where part of the DNA within the gene is different from what it should be. For example, there may be an extra part, or one that’s missing. Mutations may affect the proper growth or development of a person and can have either positive or negative effects on an individual.


The control centre of a cell. Chromosomes and therefore DNA are found within the nucleus.


The likelihood that a gene will actually result in a genetic condition.


Observable characteristics or traits, such as what you look like. The phenotype is the physical manifestation of the genotype.

Polygenic disorders

Genetic disorders resulting from the combined action of alleles of more than one gene (e.g. heart disease, some cancers, and diabetes). Although these are inherited disorders, they depend on the simultaneous presence of several alleles. This means that hereditary patterns are usually more complex than those of single-gene disorders. Compare single-gene disorders.


Long chains of amino acids. Several arranged together form a protein.


A tendency or inclination towards something, determined in advance.

Pre-symptomatic diagnosis

Diagnosis of a genetic condition before the appearance of symptoms.


The likelihood of a specific event occurring, usually expressed as a decimal fraction.


The prediction of the course and probable outcome of a condition or disease.


One of the basic chemicals of life, proteins make up the structure and determine the function of a cell.

Recessive allele

This allele is only expressed if there are two identical ones present.

Sex chromosomes

The two chromosomes, X or Y, in each cell which determine the genetic gender of an individual. Males have an X and a Y chromosome and females have two X chromosomes.

Sex determination

The mechanism by which sex is determined. In many species, sex is determined by the nature of the sperm that fertilizes the egg.

Single gene disorder

A hereditary disorder caused by a mutant allele of a single gene (e.g. Duchenne muscular dystrophy, sickle cell disease). Compare polygenic disorders.

Stem cells

Cells in the body that are capable of renewing themselves and becoming any number of different cell types.  Stem cells are made in the bone marrow. Bone marrow, and the stem cells within it, can be transplanted from a healthy donor into a patient who can’t make their own white blood cells to fight infection, for example. The stem cells can then become white blood cells, which work to fight off infection.


A group of symptoms that helps a doctor diagnose a particular condition or disease.  For example, an oddly-shaped head, slanting eyes, a single crease on the palm, and delayed mental and social skills are all symptoms of Down Syndrome.


Syndrome Without A Name. Despite scientific advances, many children and their families live without a definite diagnosis. Every case is different and it isn’t know how each will progress. The origin of SWAN may be genetic but the exact cause is not known.


An agent that raises the incidence of congenital malformations, e.g. nicotine in tobacco, alcohol, some prescription drugs such as tetracycline antimicrobials.


Any distinguishing characteristic or quality, a detectible phenotypic (physically noticeable) property of an organism.


Usually chromosomes come in pairs, one from each parent. Downs Syndrome occurs when two copes of chromosome 21 are provided by the mother and one from the father, resulting in a chromosome triplet. This is a trisomy, when there are three chromosomes instead of two.

X chromosome

Everyone has an X chromosome. If you have two you’re a girl, if you have one X and one Y you are a boy.

X-linked recessive inheritance

Boys only have one X chromosome. If it contains a faulty gene, there is effectively no back-up. Even if the gene is recessive, the corresponding disorder is expressed. Girls inherit one to become a carrier but need two copies of the faulty gene to become affected. When the disorder is caused by a recessive gene located on the X chromosome, it is X-linked recessive