Click on the index above to see definitions of terms.
A glossary of terms can be valuable for families and professionals with little or no background in genetics and who may not be familiar with the language used in the descriptions and of the genetic basis of some conditions.
The glossary is a guide to the more common terms that may be encountered but readers are advised that the local Genetics Service is a resource for any enquiries regarding genetic conditions.
Any word highlighted within the definitions is explained elsewhere in the Glossary.
Any change from the 'correct' or 'usual'. It may not necessarily mean harmful or undesirable; it can equally mean atypical, unusual or uncommon. When used in reference to, an abnormal gene may result in a genetic condition
Achondroplasia is a genetic condition that results in abnormally short stature (skeletal dysplasia). Achondroplasia is the most common cause of short stature with disproportionately short limbs (see Genetics Fact Sheet 38).
Acrocentric are those with the very close to one end, giving the chromosome a 'V' shaped appearance. The acrocentric chromosomes are numbers 13, 14, 15, 21 and 22. The short 'p' arms are very short and usually have small round appendages on stalks, known as ' '.
ADULT STEM CELL:
An undifferentiated cell found in a differentiated tissue that can renew itself and (with certain limitations) differentiate to yield all the specialised cell types of the tissue from which it originated.
ALZHEIMER DISEASE (AD):
Is the most common form of dementia. Individuals with dementia have a gradual build up of changes in the brain. The condition is characterised by progressive difficulties with memory, language, learning, thinking and reasoning, and undertaking everyday tasks. Changes in personality may also occur. There may be a genetic component to the cause of the condition (see Genetics Fact Sheet 45).
A procedure for obtaining amniotic fluid for prenatal diagnosis. Using a sterile needle, a sample of amniotic fluid is removed from the ; the amniotic fluid contains cells from the fetus which can be analysed to determine if the fetus has a specific condition. The test may be carried out from the 15th week of pregnancy (see Genetics Fact Sheet 17C).
The autism spectrum disorders (ASDs) represent a group of developmental conditions characterised by impairments in communication and social interactions, restricted, repetitive and stereotyped behaviour and anxiety and compulsions. The vast majority (about 90% of cases) are of unknown cause. There may be a genetic component in some cases (see Genetics Fact Sheet 43).
Any that is not a sex chromosome (that is not an X or Y chromosome). In humans, the autosomes are the numbered chromosomes and are given the numbers 1-22. Chromosome 1 is the largest and 22 is the smallest.
BALANCED TRANSLOCATION (RECIPROCAL TRANSLOCATION):
Please see 'Translocation'.
A series of darkly and lightly stained stripes across a are produced by treating the chromosomes with different chemicals. There are a number of different staining techniques which produce different patterns eg. , R-banding etc.
The sex chromatin or highly condensed inactive X chromosome present in each somatic cell in females (see Genetics Fact Sheet 14).
Also known as nucleotides, they are the basic components of DNA. They are denoted by the letters A (Adenine), G (Guanine), C (Cytosine) and T (Thymine). The sequence of these bases forms the genetic code.
BECKER MUSCULAR DYSTROPHY (BMD):
Is a genetic muscle condition usually affecting boys. Symptoms usually start in adolescence or adulthood and progress slowly but will affect all voluntary muscles.It is characterised by generalised weakness and muscle wasting that affects limb and trunk muscles first. Similar to but less severe (see Genetics Fact Sheet 41).
A 5-6 day old embryo, ready for implantation to occur. The blastocyst consists of a sphere made up of an outer layer of cells (the trophectoderm), a fluid-filled cavity (the blastocoel), and a cluster of cells on the interior (the inner cell mass).
The result of uncontrolled cell division and growth. Cancer occurs when cells in the body become abnormal due to changes in genes, and consequently these cells grow out of control. Cancer cells have the ability to spread to other parts of the body (metastasise; see Genetics Fact Sheets 47, 48, 49, 50 and 51).
CARRIER OF A CHROMOSOMAL REARRANGEMENT:
This definition applies to an individual who has a rearrangement of his/her chromosomes so that the normal genetic information is present (that is, it is 'balanced') but it is not in the usual 46 chromosome pattern.
CARRIER OF A MUTATED GENE:
Every contains two copies of each . One gene copy may be and the other may be 'correct'. If the mutated gene is not expressed in the cells (resulting in a particular characteristic or a condition), the mutated gene is said to be to the other 'correct' copy of the gene. An individual who has one correct gene copy and one faulty (recessive) gene copy is said to be a 'carrier' for the mutation leading to a specific condition. The carriers of a recessive mutation in a gene are usually not affected but they are at risk for passing on the faulty gene to their offspring.
The basic structural unit of all living organisms. While some organisms are made up of only one or several cells, humans are composed of millions of cells. Each cell is enclosed by a membrane and has a nucleus which contains the genetic material (DNA) in the form of chromosomes. Mitochondria are also found randomly scattered throughout the cell.
The mechanism by which cells multiply during the growth of tissues or organs. The type of cell division involved in the growth of the body is known as mitosis. The cell division which produces sperm or ova (an egg) in the testis or ovary is known as meiosis.
The constricted part of the chromosomes which separates it into its two arms. The short arm is called the 'p' arm (for 'petite'); the long arm is called the 'q' arm (because q follows p in the alphabet).
The chorion develops into the placenta. Chorionic cells have the same genetic composition as cells of the fetus. Cells of the chorion are sampled during a prenatal diagnostic test called CVS (chorionic villus sampling).
CHORIONIC VILLUS SAMPLING (CVS)
A procedure for obtaining cells of the chorion to enable testing of the for specific abnormalities. Samples of the cells may be taken through the vagina or through the abdomen of the pregnant mother. It is usually carried out in the 10th-12th week of pregnancy (see Genetics Fact Sheet 17C).
A threadlike structure found in the nucleus of all the body (except red blood cells) consisting of and . Each chromosome can be thought of as a string of beads where every bead represents a gene (see Genetics Fact Sheet 1).
Please see ' '.
Please see ' '.
Also know as thrombophilia, describes an increased tendency for clots to form. Some individuals have inherited a genetic susceptibility to develop blood clots within a blood vessel, and therefore have an inherited clotting condition (see Genetics Fact Sheet 39).
The equal expression of both copies of ain an individual eg. presence of both haemoglobin A and S on in an individual for sickle-cell disease.
Three-letter words composed of combinations of the chemical letters A, G, C and T. These words make up the genetic code (DNA) that tells the cell to make a product (protein) that the cells can use. Also known as nucleotide triplets.
Patterns of inheritance that differ from the ‘traditional’ (Mendelian) patterns of inheritance in that they require multiple factors (either genetic or environmental) for the condition to develop.
CONGENITAL HEART DEFECTS:
Abnormalities of the structure and/or function of the heart that are present at birth that may be so slight that the baby appears healthy for many years after birth; others are so severe that they are life threatening (see Genetics Fact Sheet 56).
A general term for all tissues of the body which support and connect various organs and other structures such as the skeleton. Certain types of connective tissue act as a glue, some as scaffolding and others permit expansion and contraction.
Relationship between two individuals with a common ancestor, for example, cousins (see Genetics Fact Sheet 16).
Removal of a blood sample from the umbilical cord under guidance for prenatal diagnosis of certain conditions. The procedure is similar to or trans-abdominal CVS (see Genetics Fact Sheet 17C).
When pairs join together during (the division process which produces the egg and sperm cells), the two chromosomes may exchange material: part of one chromosome 'crosses over' and exchanges places with the corresponding part on its partner chromosome.
The broth that covers cells in a laboratory environment. The culture medium contains nutrients to feed the cells and may encourage them to multiply. Other factors may be added to culture in order to direct desired changes in the cells.
Please see .
CYSTIC FIBROSIS (CF):
Is a genetic condition that affects many organs in the body, especially the lungs, pancreas and sweat glands. A build-up of thick, sticky mucus in these organs leads to respiratory problems, incomplete digestion and increased salt loss from the sweat glands. As a result of early diagnosis and treatment, 50% of those with CF now live into their late 30s but the condition can severely affect their quality of life (see Genetics Fact Sheet 33).
DEAFNESS (HEARING LOSS):
May be conductive (structural problems within the ear affecting hearing) or sensorineural (problems with the tiny sensory hair cells in the cochlea, or the auditory nerve, or in the processing of sound in the brain). Deafness may be inherited (see Genetics Fact Sheet 60).
The loss of some genetic material from a . If the deletion is large, it may be observed in the karyotype as missing chromosomal material. If it is small, it may only be detected by analysing the composition of the .
Please see .
DIABETES MELLITUS (DIABETES):
A condition resulting in high levels of sugar (glucose) in the blood. There are several types of diabetes mellitus and some may have a genetic component (see Genetics Fact Sheet 57).
DIAGNOSTIC AND STATISTICAL MANUAL OF MENTAL CONDITIONS:
A handbook used by mental health professionals to diagnose mental conditions.
A term used to describe particular tests that are able to identify (diagnose) a recognised condition. Prenatal diagnostic tests include; CVS, amniocentesis and some ultrasound imaging.
DNA (DEOXYRIBONUCLEIC ACID):
The chemical compound that makes up within s and is the basic material of heredity. It is made up of chemicals called nucleotide bases, linked together in a chain. Two chains of nucleotides twist around each other to form a double helix.
The pattern of sequences of DNA base pairs at certain sites on a chromosome. The DNA fingerprint is as unique as possible for each individual. Specialised DNA testing to determine an individual's DNA fingerprint may be used as evidence in criminal law cases (see Genetics Fact Sheet 22).
Every cell contains two copies of each . Where only one of the gene copies or is mutated, and the other allele is 'correct', but the person is affected by a genetic condition due to that , the mutation is described as dominant. The mutated gene is said to be dominant over the other 'correct' copy of the gene. A condition or characteristic caused by a dominant gene mutation only requires one of the genes to be mutated for the person to be affected.
Also known as trisomy 21, is a chromosomal condition that is most commonly the result of a whole extra copy of chromosome 21 found in all cells of the body. Characteristics may include intellectual delay, distinct facial features and problems with heart and digestive tract (see Genetics Fact Sheet 28).
DUCHENNE MUSCULAR DYSTROPHY (DMD):
A genetic condition that causes severe weakness, delayed walking and other problems. It usually occurs in boys (see Genetics Fact Sheet 41).
A part of the is present in two or more copies. If the duplication is large it may be observed under the microscope as a change in a chromosome; a small duplication may only be observed by examining the structure of the chromosome or a .
Comes from the Greek words DYS - meaning abnormal, disease, faulty, impaired, and MORPHOLOGY - meaning structure or form. Refers to the changes in the usual structure of a person's , in particular, external appearance.
Also known as trisomy 18, is a chromosomal condition that is the result of an extra copy of chromosome 18. It is a severe condition where children with the condition may have small eyes, abnormally formed ears, heart defects and severe intellectual impairment. They rarely live past one year (see Genetics Fact Sheet 30).
The female sex cell (ovum) which carries half the mother's chromosomes (and therefore, half the genes). In humans, this number is normally 23. The egg joins with the sperm at conception to produce an embryo.This process is called fertilisation.
A protein molecule which promotes or enables a chemical reaction in the cells (a biochemical reaction) to take place. These biochemical reactions include breaking down food into the essential chemicals required by the body and breaking down toxic by-products of our bodies. Enzymes are essential for the correct function of the body's metabolism.
ENZYME REPLACEMENT THERAPY:
A method of treating genetic conditions that are due to a deficiency of a particular enzyme. Overcoming the deficiency by providing the body with the enzyme enables the cells to function correctly and the symptoms of the condition may be reduced or eliminated.
Making heritable changes in the way that a gene works or functions, without altering the sequence of the genetic code, or DNA. Ways in which this may be changed is via structural changes (see Genetics Fact Sheets 14 and ).
The degree to which an inherited characteristic is expressed in a person. 'Variable expressivity' refers to the variation in expression and severity of particular characteristics or severity of a condition.
FAMILIAL HYPERCHOLESTEROLAEMIA (FH):
An inherited tendency to have high cholesterol, which may lead to coronary artery disease (see Genetics Fact Sheet 53).
FAMILIAL HYPERTROPHIC CARDIOMYOPATHY (FHCM):
A condition in which part of the heart muscle surrounding the ventricles (lower chambers) - in particular the left - is thicker than normal. This may cause problems such as palpitations, breathlessness and chest pain, but some people are symptom-free. There may be a genetic component to the condition (see Genetics Fact Sheet 54).
A record of the health conditions of different members within a family, across generations. It may be recorded as a list or as a pedigree that identifies the structure and relationship within the family. It provides a view of conditions or illnesses within a family to allow for analysis of inheritance patterns.
A small break or a constriction of a chromosome that can be visualised after special treatment of the chromosomes. In individuals affected with Fragile X syndrome, a fragile site can often be seen on their X chromosome (see Genetics Fact Sheet 42).
FRAGILE X SYNDROME:
A genetic condition usually affecting boys. It is characterised by particular physical features, varying degrees of learning difficulties and behavioural and emotional problems (see Genetics Fact Sheet 42).
Banding patterns on chromosomes make it easier to examine the chromosomes under the microscope for abnormalities in structure and number. G bands are one type of banding pattern, induced to appear on chromosomes by staining them with a special chemical called Giemsa.
Treating genetic conditions and diseases by replacing, manipulating or supplementing genes that are not working properly (see Genetics Fact Sheet 27).
The information contained in the DNA which is 'interpreted' by the cells to produce proteins. The chemicals (nucleotides) which make up the DNA can be described by the letters A (Adenine), T (Thymine), C (Cytosine) and G (Guanine). Thus the genetic code can be written as a series of letters (for example AAA CGT TTC).
A genetic condition is caused by a change in the genetic information. Genetic conditions may be caused by a mutation in a single gene or may be caused by a change in chromosome structure or number (see Genetics Fact Sheet 2).
The provision of diagnosis, information and support by health professionals with specialised training in genetics and counselling (see Genetics Fact Sheet 3).
A health professional with specialised training in genetics and counselling who can provide information and support to individuals or families with concerns about a genetic condition which may run in their family.
Analysis of an individual's genetic make-up to determine predisposition to a particular health condition or to confirm a diagnosis of a genetic condition. Other applications for genetic testing include forensic DNA analysis (used in criminal investigations) and paternity testing (to determine the father of an individual) (see Genetics Fact Sheets 21 and 22).
A condition in which the amount of iron in the body is much higher than usual. The iron builds up over time in various organs such as the liver, heart and brain. The condition can be associated with another medical problem or it may be inherited (hereditary haemochromatosis). If untreated, the build-up of iron may lead to conditions such as arthritis, cirrhosis of the liver, cardiomyopathy and diabetes (see Genetics Fact Sheet 36).
A genetic condition that causes people to continue bleeding for a long time unless treated. Bleeding is common into joints such as knees, ankles and elbows. Sometimes bleeding into muscles or internal organs may occur. The blood is unable to clot properly due to missing or abnormal proteins that facilitate clotting (see Genetics Fact Sheet 40).
A condition most likely due to the interaction of changes in a number of different genes that leads to a susceptibility to the condition, triggered by environmental factors that may include diet, obesity and stress (see Genetics Fact Sheet 56).
An individual who has two different alleles at a particular gene locus, one on each chromosome of a pair. One allele is usually normal and the other abnormal. Such an individual may also be referred to as a carrier.
Refers to an individual in whom the two alleles or gene copies contain identical information. An individual can be homozygous for the correct copies of the gene or can be homozygous for the mutated copies of the gene.
HUMAN GENOME PROJECT (HGP):
The international scientific effort that began in the 1980s to 'read' the order of bases (sequence) as they appear in the DNA of human chromosomes. The objective is to create a directory of the genes that can be used to answer questions such as what specific genes do and how they work (see Genetics Fact Sheet 24).
HUNTINGTON DISEASE (HD):
A neurodegenerative genetic condition where the symptoms of involuntary movements and changes in personality usually appear slowly between the ages of 30 and 50 (see Genetics Fact Sheet 44).
The 'memory' held by a chromosome as to whether it was inherited from the mother or the father. The memory is chemically 'stamped' into the DNA and can result in chromosomes or the genes located on the chromosomes behaving differently, depending on the parent of origin.
INBORN ERROR OF METABOLISM:
A congenital condition which results from a change in a gene which causes a deficiency in the presence or activity of particular enzymes important for the functioning of the body's metabolism (see Genetics Fact Sheet 20).
The number of new cases of a condition, detected annually, per unit of the population. For genetic conditions, the incidence is quoted as the number of affected individuals per 1,000 births whether detected at birth or not.
Please see 'Penetrance'.
The transmission of genetic information from a parent to a child.
The addition of a piece of chromosomal material into a chromosome in a place where it is not normally found. This may result in a condition, because the genetic code may then be read or translated incorrectly.
Where there are two breaks in a chromosome, the segment may flip over and rejoin, that is, become inverted. This results in the genes being in the reverse order along the chromosome. This may cause the genetic code to be read or translated incorrectly.
Please see ' '.
There are no terms for this section.
The term used to describe an individual's chromosomes that have been photographed through the microscope and then arranged according to a standard classification based on their group and size.
A genetic condition that results in excessive copies of the X chromosome (usually two) in a male (47,XXY), instead of the usual chromosome set for males (46,XY). Many males with Klinefelter syndrome are intellectually impaired, have underdeveloped testes and infertility (inability to father children), and may be taller than average (see Genetics Fact Sheet 31).
LONG QT SYNDROME (LQTS):
A heart condition that may have a genetic component. In individuals with long QT syndrome, it takes longer for the electrical signal to activate and inactivate the lower chambers of their heart (the ventricles); described as a longer Q-T interval on the ECG (electrocardiogram) - they have long QT syndrome (LQTS; see Genetics Fact Sheet 55).
MATERNAL SERUM TESTING:
A test that assesses the risk of foetal abnormalities such as neural tube defects and Down syndrome by analysing a number of biochemicals in the mother's blood during pregnancy (see Genetics Fact Sheet 17B).
This refers to the inheritance of single genes and follows specific patterns: autosomal dominant, autosomal recessive and X- linked inheritance (see Genetics Fact Sheets , and ).
Conditions included under the 'umbrella' term of mental illness are schizophrenia, depression and bipolar disorder (manic depression). There may be an inherited susceptibility (see Genetics Fact Sheet 58).
A genetic test usually done by obtaining a blood sample that looks for extra or missing DNA segments along chromosomes. Having more or less DNA than usual can cause health or developmental concerns. (see Genetics Fact Sheet 6a)
These structures or organelles in the cell are the main energy source and are often called the powerhouse of the cell. The mitochondria also contain their own DNA and therefore genes. Mitochondrial genes follow maternal inheritance (see Genetics Fact Sheet 12).
Where one chromosome is represented once only instead of twice, e.g. girls with Turner syndrome have only one X chromosome instead of the usual two copies (Monosomy X; see Genetics Fact Sheet 32).
Please see 'Identical twins'.
A situation where some cells have an abnormal or unusual genetic/chromosomal make-up while the rest of the cells in the body have the usual genetic/chromosomal constitution. For example, a person who is mosaic for trisomy 21 (see Down syndrome) would have some cells which have 47 chromosomes with an extra chromosome number 21 and other body cells which have the usual 46 chromosome complement. The number of cells with abnormal genetic or chromosomal content will determine the level of severity of the condition.
A pattern of inheritance which results from the interaction of one or more genes with environmental factor(s) (see Genetics Fact Sheet 11).
A permanent change in a gene. If the mutation occurs in the germ line cells, it is then able to be inherited. Mutations in somatic cells cannot be inherited. Mutations can occur naturally and spontaneously or they may be due to exposure to mutagens.
NEURAL TUBE DEFECTS (NTDs):
Problems in the development of the spinal cord and brain, such as the failure of the spine to enclose the spinal cord (spina bifida) and the failure of the brain to develop (anencephaly) (see Genetics Fact Sheet 59).
NEUROFIBROMATOSIS TYPE 1 (NF1):
A genetic condition where features include flat coffee-coloured 'birth marks' on the skin (cafe-au-lait patches); neurofibromas (harmless soft pink small lumps that can grow on nerves anywhere in the body); freckles in areas that are not usually exposed to sunlight; Lisch nodules (harmless small brown spots on the iris of the eye) (see Genetics Fact Sheet 37).
NEUROFIBROMATOSIS TYPE 2 (NF2):
A genetic condition characterised by the development of swellings (non-cancerous tumours called schwannomas) on the nerves that control hearing and balance (auditory and vestibular nerves) (see Genetics Fact Sheet 52).
NEWBORN SCREENING (NBS):
A public health funded system for testing newborn babies' blood for about 30 rare conditions. Some of these conditions can result in physical and/or intellectual problems if not treated promptly and are often referred to as inborn errors of metabolism (see Genetics Fact Sheet 20).
Where the chromosome pairs fail to separate correctly in meiosis, resulting in sperm or egg cells which have missing or extra chromosomes e.g. if chromosome number 21 fails to separate in the formation of an egg (or sperm), one egg (or sperm) will contain an extra copy of chromosome 21 (24 chromosomes) while the other egg (or sperm) will contain only 22 chromosomes (see Genetics Fact Sheet 6).
OBLIGATE MUTATION CARRIER:
A family member who is clincally unaffected with a genetic condition but on the basis of their pedigree (family health tree), must be a carrier of a mutated gene for that condition.
Structures within cells such as the nucleus, mitochondria and lysosomes which have special functions.
Each chromosome is divided into two parts, joined by the centromere. The 'p' arm is the shorter of the two segments and is at the top of the chromosome. The longer segment is called the 'q' arm.
PARKINSON DISEASE (PD):
The second most common adult-onset neurological condition with three major symptoms: tremor, stiffness of the muscles and slowed movements. There may be a genetic component to the cause of the condition (see Genetics Fact Sheet 46).
Also known as trisomy 13, is a chromosomal condition that is the result of an extra copy of chromosome 13. It is a severe condition where children with the condition may have small wide-set eyes, cleft lip and palate, extra fingers and toes, scalp defects, malformed low-set ears, microcephaly (smaller head), severe intellectual impairment, kidney defects and heart defects (see Genetics Fact Sheet 29).
The probability of detecting the presence or clinical expression of a gene or combination of genes when they are present. If the penetrance of a particular condition is less than 100%, not all individuals who carry a mutation in the gene or genes responsible for the condition will develop symptoms of the condition it causes. Such a genetic condition is said to have reduced or incomplete penetrance.
The study of how an individual's genetic make-up affects their response to medicines. The terms pharmacogenomics and pharmacogenetics tend to be used interchangeably. Pharmacogenetics is generally thought of as the study of genetic differences that give rise to differing responses to medications and drugs, while pharmacogenomics is the broader application of genomic technologies to new drug discovery and further characterisation of older drugs. Pharmacogenetics considers one or at most, a few, genes of interest, while pharmacogenomics considers the entire genome (see Genetics Fact Sheet 25).
A form of genetic testing performed on a person with a family history of a particular genetic condition, but who does not have any symptoms of the condition at the time of testing. This testing determines if that person has inherited the mutation (present in their family). If testing for this mutation reveals that it is present in the person, then they have an increased predisposition to developing the condition that was tested for. The detection of a specific mutation does not necessarily mean the individual will definitely develop the condition. Familial breast cancer is an example of a condition where predictive testing is used (see Genetics Fact Sheet 48).
Tests during pregnancy to assess the possibility that a foetus is affected with a particular condition. These tests are not 100% accurate (see Genetics Fact Sheet 17B ).
A form of genetic testing performed on a person with a family history of a particular genetic condition, but who does not have any symptoms of the condition at the time of testing. This testing determines if that person has inherited the mutation (present in their family). If testing for this mutation reveals that it is present in the person, then they will most likely develop symptoms of the genetic condition it causes at some stage of their life. Huntington disease is an example of a genetic condition where presymptomatic testing is used (see Genetics Fact Sheet 44).
A small segment of DNA of known origin, manufactured in the laboratory, which is designed to recognise the DNA on specific parts of chromosomes. A coloured stain can be attached to the probe and used to confirm the presence or absence of a particular gene or mutation.
A surgical procedure designed to prevent the development of a disease. For example, prophylactic mastectomy is the removal of one or both breasts in an effort to prevent breast cancer in individuals who carry a mutation in a breast cancer gene and are therefore at high risk of developing breast cancer.
Genes that are part of a person's usual genetic make-up. They have a role in various aspects of cell division. If these genes are changed in some way, they may give rise to oncogenes that can lead to cancer.
Every cell contains two copies of each gene. Each gene contains the information for a particular gene product, such as a protein. If a gene is mutated, the gene no longer codes for the gene product. Where an individual has one gene copy or allele mutated and the other copy 'correct', the cell will only be producing half the amount of gene product. If this does not result in any condition for the individual, the mutation is described as being hidden or 'recessive' to the correct copy of the gene. An individual with this genetic constitution is said to be a 'carrier' of a recessive gene mutation. For a recessive gene mutation to result in a particular characteristic or a condition, both copies of the genes must be mutated (see Genetics Fact Sheet 8).
Chance that a specific process or event may occur. 'Genetic risk' refers to the likelihood or probability that a genetic characteristic or condition will occur or recur in a family, based on an understanding of the pattern of inheritance.
RFLPs (RESTRICTION FRAGMENT LENGTH POLYMORPHISMS):
The fragments of DNA which result when it is cut by special enzymes called restriction enzymes. The patterns of these fragment lengths are used to indicate the presence or absence of mutations in particular genes.
Please see 'RNA'.
This occurs as a result of the fusion of the two ends of the same chromosome; there is a consequent loss of genetic material.
A type of translocation exclusive to the acrocentric chromosomes (13, 14, 15, 21 and 22) in which two of these chromosomes join at or near their centromeres. This is effectively a fusion between two whole chromosomes.
Small, round appendages attached by fine stalks to the ends of the short 'p' arms of the acrocentric chromosomes. They do not always stain darkly and may be difficult to see in a karyotype.
SHORT TANDEM REPEATS (STRs):
Multiple copies of a short, identical DNA sequence repeated in tandem. The pattern of different numbers of STRs at certain sites on the chromosomes may be used to create a DNA pattern or DNA fingerprint that is as unique as possible for each person.
SICKLE CELL DISEASE:
An inherited blood condition in which the haemoglobin molecule (a type of protein in red blood cells that carries oxygen to the tissues) is abnormal. This produces a severe form of anaemia (see Genetics Fact Sheet 34).
SINGLE NUCLEIOTIDE POLYMORPHISM (SNP):
A DNA sequence variation that involves a change in a single nucleotide. Variations in the genetic code at the level of one nucleotide may be useful in certain applications such as assessing the patterns of inheritance via genetic linkage studies, or forensic DNA testing or DNA fingerprinting.
A large group of genetic conditions in which the bony skeleton develops abnormally. An example of a skeletal dysplasia is achondroplasia (see Genetics Fact Sheet 38).
All the cells of the body except the reproductive cells (sex cells).
SOMATIC GENE THERAPY:
Correcting or transplanting genes that are present in somatic cells, not the sex cells.
A change or fault in a gene which is found in the cells of the body but not in the germ or sex cells. Somatic mutations cannot therefore be passed on to future generations.
SOMATIC STEM CELLS:
Another name for adult stem cells.
The ability of a test to determine whether a genetic condition or mutation is absent when it is truly absent.
SPERM (ABBREVIATION OF SPERMATOZOAN):
The male sex cell which contains 23 chromosomes. It fuses with the egg during fertilisation.
A mutation that results in a genetic condition and which appears for the first time in a family. The mutation takes place in either the egg or the sperm or at conception.
Undifferentiated cells with the ability to divide for indefinite periods in cell culture and to give rise to specialised, functional cells.
Repetitive routines or movements that have no function or use.
Any of three mRNA sequences (UGA, UAG, UAA) that do not code for an amino acid and therefore signal that protein production or translation stops.
Any disease or condition which is characterised by the abnormal accumulation and storage of material within the cells. The stored material will vary depending upon the type of condition.
A gene in which, if there is a change or mutation, increases an individual's susceptibility or predisposition to a certain disease or condition. When such a gene change is inherited, there is a greater likelihood that symptoms will develop, but this is not certain.
A group of characteristics and/or symptoms that occur together in a recognisable pattern.
The process of building a complex compound from a number of simple chemical elements.
TAY SACHS DISEASE (TSD):
An inherited degenerative condition of the nervous system where apparently healthy babies at about six months of age lose already acquired skills, gradually become blind, paralysed and unaware of his/her surroundings and die before approximately five years of age. It is more common amongst people of Ashkenazi Jewish background (see Genetics Fact Sheet 35).
The terminal or end segment of each chromosome arm.
A substance that produces or increases the incidence of birth defects or congenital abnormalities by interfering with development of the foetus during pregnancy.
TERMINATION OF PREGNANCY:
Intervention to ensure a pregnancy does not continue.
Four copies of every chromosome resulting in 92 chromosomes in the cell, instead of the usual 46.
Four copies of a particular chromosome present in a cell, resulting in 48 chromosomes in the cell instead of the usual 46.
An inherited blood condition in which the chains of the heamoglobin molecule (a type of protein in red blood cells that carries oxygen to the tissues) are decreased. Alpha thalassaemia is where the gene for the alpha chain is not working properly, while beta thalassaemia is where the beta chain gene is faulty. Signs and symptoms of the thalassaemias vary from mild (little to no symptoms) to severe (life threatening) (see Genetics Fact Sheet 34).
A clot in a blood vessel obstructing the flow of blood.
Please see 'Clotting condition'.
A blood clot.
One of the four basic building blocks (nucleotide bases) that makes up the genetic code (DNA). It is represented by the letter T.
Collection of interconnected cells that have a similar function in the body.
Making an RNA copy from a sequence of DNA (a gene). Transcription is the first step in producing a gene product or protein.
A gene artificially introduced into the cell or into the genome of an individual.
The process of producing a protein from mRNA by stringing together amino acids (protein chemical building blocks) in a specific order, and this is determined by the mRNA code. Translation is the second step in creating a protein from DNA, the first step being transcription.
This occurs when a piece of one chromosome breaks off and attaches to another, different chromosome. When no material is lost or gained the translocation is said to be 'balanced' and the individual may or may not be affected by it. An 'unbalanced' translocation results in the loss or gain of genetic material which may result in a genetic condition.
Please see 'Triplet Repeat'.
A sequence of three nucleotides in the DNA sequence. Each triplet represents the code for a particular amino acid.
TRIPLET REPEAT (TRINUCLEOTIDE REPEAT):
A form of genetic mutation which consists of a series of repeated identical sequences of DNA triplets which may be found either inside or outside a gene. An increase in the number of such repeats in a particular gene can lead to instability of the gene and manifestation of the corresponding genetic condition. This form of genetic mutation has been associated with a number of genetic conditions including Huntington disease, Fragile X syndrome, Myotonic dystrophy etc.
Having three copies of every chromosome resulting in 69 chromosomes in a cell instead of the usual 46.
Three copies of a particular chromosome are present in a cell resulting in 47 chromosomes instead of the usual 46.
Please see 'Patau syndrome'.
Please see 'Edward syndrome'.
Please see 'Down syndrome'.
TUMOUR SUPPRESSOR GENE:
A gene which contains the information for proteins whose role in cells is to suppress the formation of tumours by controlling cell division and growth. Loss of this control leads to development of malignancy. P53 is an example of a tumour suppressor gene.
Also called X0 syndrome, Turner syndrome is a chromosomal condition where females are missing one (or part of one) of their two X chromosome copies. They have a total of 45 chromosomes including one X xhromosome (45,X) instead of the ususal two copies (46,XX). This condition is characterised by short stature and the lack of sexual development in girls at puberty. Other physical features may include a short neck with a webbed appearance, heart defects and kideny abnormalities (see Genetics Fact Sheet 32).
The use of sound waves for visualising body tissues and structures. In pregnancy, structural abnormalities in the foetus can be detected (see Genetics Fact Sheet 17A).
A cell that has not matured into a functional, specialised cell type.
Where both members of a chromosome pair are contributed by one parent rather than one from each parent. Uniparental disomy may be maternal or paternal.
The female sexual organ in which the foetus develops into a baby over a period of nine months.
The normal process in which one X chromosome in each body cell of a female is randomly inactivated. The genes on the inactive X chromosome are 'switched off' or not working. This is also known as lyonisation, and is a form of imprinting. The inactive X chromosome may be seen as a Barr body under the microscope (see Genetics Fact Sheet 14).
Any gene which is located on the X chromosome.
The single cell with 46 chromosomes resulting from the fertilisation of an egg (containing 23) by a sperm (containing 23). Through cell division (mitosis), the zygote develops into a multicellular embryo and then into a foetus.