The following list provides useful definitions to many of the technical terms used throughout this website.

The list is not fully inclusive.  Any information on this website is provided for education and information purposes only and is not a substitute for personal, medical or genetic advice. Families should consult a medically qualified clinician in all matters relating to genetic diagnosis, management and health.

Medical information changes rapidly and whilst Jnetics make a great effort to ensure that the information is accurate and to update site content, some information may be out of date. The website should not be relied upon as a substitute for consultation with your doctor or other qualified health care professional.

Ashkenazi Jews

Jewish people of Eastern/Central European ancestry.

Autosomal recessive

A pattern of inheritance in which a disorder results only when an individual inherits two copies of a specific disorder-related mutation, one from each parent. With recessive conditions a single fully functional copy of a gene is sufficient to compensate for the mutated copy, preventing the disorder developing. Where both partners of a couple are carriers for the same disorder gene, there is a 1 in 4 chance in each pregnancy of a child being affected.

Autosomal dominant

A pattern of inheritance where an alteration in just one copy of a pair of genes is sufficient to cause a detectable trait. Individuals who have one mutated copy of a gene for a dominant disorder are likely to be affected. Where one partner of a couple has an abnormal copy of a gene for a dominant disorder, there is a 1 in 2 chance in each pregnancy of a child inheriting the abnormal gene and potentially be affected by the disorder.


The chromosomes that are not concerned with sex determination. Humans have 22 pairs of autosomes, plus two sex chromosomes.

Benign tumour

A tumor that does not spread to other parts of the body and does not grow back once it has been removed.


A person who has one altered (mutated) copy of a gene and one normal copy of the same gene. For recessive disorders, carriers are unaffected by the disorder but are at risk of passing it on to their children.

Carrier frequency

Number that describes how many people in a population are carriers (have one altered copy of a gene and one normal copy of the same gene) for a genetic disorder. Carriers for recessive disorders are unaffected but are at risk of passing on the disorder to their children.


A cell is the smallest structural and functional unit of an organism. Cells are microscopic and are often referred to as the ‘building blocks of life’.


The structures within the nucleus of every human cell, within which genes are packaged. Each chromosome is composed of a single, long molecule of DNA, complexed with specific proteins. A normal human cell contains 23 pairs of chromosomes, 22 pairs of autosomes and 2 sex chromosomes.


Any trait or condition that is present at birth. The condition or trait usually exists before birth, but can be caused by damage during the birthing process.


Deoxyribonucleic Acid (DNA) is a molecule that encodes the genetic information responsible for the structure and function of living organisms. It allows for the transmission of inherited information from generation to generation.

Dominant inheritance

See Autosomal dominant


A protein that facilitates a specific chemical reaction within a cell, without being changed itself. Enzymes are required for the normal metabolism (breakdown) of substances in the body.


The extent to which a genetic disorder is expressed. When there is variable expressivity of a disorder, affected individuals may have symptoms ranging from mild to severe, but will always have some symptoms. This contrasts with penetrance, in which the disorder is either present or not (all-or-none expression).


The stage of human development from 10 weeks gestation until birth.

Founder Effect

This can occur when a new population is established by a few original founders who only carry a small fraction of the total genetic variation of the original, larger population. If one or more of these founding member carry a disease mutation, then a significant proportion of future descendants may also carry the mutation and the gene frequency would then be much higher than it was in the general population that the settlers come from.


The basic unit of inheritance. Each gene contains instructions for making a specific protein, and each protein has a particular function in the body.

Genetic counsellor

A person qualified to help families at risk of, or affected by, genetic disorders to make informed decisions about genetic testing and family planning.

Genetic counselling

A communication process which deals with human problems associated with the occurrence or the risk of occurrence, of a genetic disorder in a family.

Genetic disorders

A condition resulting from an alteration, or mutation, in one or both copies of a specific gene, leading to the development of symptoms.

Genetic drift

Genetic drift happens due to random changes in how often a gene appears in a population. Whilst drift appears in all populations, its effects are more evident in very small populations where certain characteristics can be seen more frequently relative to the general population.

Gene therapy

Treating a disorder by replacing, manipulating, or supplementing non-functional genes with functional genes.


The scientific study of heredity, or how particular qualities traits or genes are transmitted from parent to offspring.

Genetic testing

The testing process used to detect the presence or absence of, or change in, a particular gene or chromosome.


The transmission of genes from parent to child.

In vitro fertilisation (IVF)

IVF is an assisted reproduction technique in which fertilisation is achieved outside of the body.

Jewish genetic disorders

A group of genetic conditions that, whilst not exclusively Jewish, have a higher prevalence among people with Jewish ancestry relative to the general population.

Lysosomal storage disorders

A group of approximately 40 rare inherited metabolic disorders that result from defects in lysosomal function. Lysosomal storage disorders are caused by lysosomal dysfunction usually as a consequence of deficiency of a single enzyme required for the metabolism of lipids, glycoproteins (sugar containing proteins) or so-called mucopolysaccharides.

Malignant tumour

A malignant tumor is a cancerous tumour, which spreads to other parts of the body.


A mammogram is an image taken by mammography. This is an X-ray technique used in breast cancer screening.

Mendelian disorder

A disorder caused by a single gene defect, which can occur in either dominant or recessive inheritance patterns. Mendelian inheritance follows the ratios first described by Gregor Mendel. Non-Mendelian disorders are those which are develop due to environmental causes or due to a combination of factors.


MRI, or magnetics resonance imaging, is a medical imagine technique that uses strong magnetic fields and radio waves to create an image of the inside of the body. It is done using a large tube-shaped MRI scanner, and an individual lies inside this tube during the scanning process.


A mutation refers to a structural change in a gene. It can be an alteration to a gene’s size, arrangement, or molecular sequence.


NICE stands for The National Institute for Health and Care Excellence. It is an independent organisation set up by the Government that decides which drugs and treatments are available on the NHS in England and Wales.


The likelihood that a person carrying a disorder-related mutation will actually manifest the disorder. Penetrance is most often associated with dominant disorders. A highly penetrant disorder is one in which the majority of individuals who have the mutated gene will develop the disorder. A disorder with reduced penetrance is one in which only a percentage of people with the mutation will ever develop the disorder.

Predisposition gene

A gene that increases the likelihood of an individual developing a disease when it is mutated.

Preimplantation genetic diagnosis

Use of genetic testing on one or two cells taken from a live early-stage embryo created by IVF. The procedure is usually carried out in order to determine whether the embryo is affected by a serious genetic disease. Unaffected embryos would be implanted in the uterus.


Before a baby is born.


A large, complex molecule composed of amino acids. The sequence of amino acids (and the function of the protein) is determined by the sequence of the genetic code. Proteins are essential to the structure, function and regulation of the body.

Recessive inheritance

See autosomal recessive.

Reduced penetrance

A disorder with reduced penetrance is one in which only a percentage of people with the mutation will ever develop the disorder.

Sephardi Jews

Descendants of the Jews who lived in Spain or Portugal before 1492. The term is now often applied to refer to anyone Jewish of non-Ashkenazi ancestry (including the ancestrally distinct group of Middle Eastern Jews.

Sex chromosomes

The chromosomes that determine the sex of an individual. Females have two X chromosomes while males have an X and a Y chromosome.

Stem cells

Primal cells found in multicellular organisms that retain the ability to renew themselves through mitosis and can differentiate into a diverse range of specialised cells.

Tumour suppressor gene

Gene that plays a role in controlling cell survival and division. If both copies of a tumour suppressor gene are deleted or inactivated, these controlling functions are lost and unrestrained proliferation may result.

X chromosome

One of the two sex chromosomes of humans. Females have two X chromosomes, while males have one X and one Y chromosome.


Genes found on the x-chromosome. A male who inherits a mutation on an X chromosome is likely to manifest the related disorder because he has no second copy of the gene to compensate. Examples of X-linked disorders include haemophilia and X-linked colour blindness.

Y chromosome

One of the two sex chromosomes of humans. Females have two X chromosomes while males have one X and one Y chromosome. The Y chromosome carries a male-sex-determining gene that initiates development as a male.