Glossary of Genetic Terminology
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ABNORMAL:
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 genes, 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:
Acrocentric chromosomes are those with the
centromere 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 'satellites'.
One of the four basic building blocks (nucleotide bases) that makes up the genetic code or DNA. It is represented by the letter A.
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.
ALLELE:
There are usually two copies of a gene. These two
copies are called alleles. In some cases, one or both, alleles will be mutated
or altered in some way.
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).
AMINO ACIDS:
Small chemical building blocks that join together to form proteins: there are 20 common amino acids which join in different combinations to make up proteins.
AMNIOCENTESIS:
A procedure for obtaining amniotic fluid for prenatal diagnosis. Using a sterile needle, a sample of amniotic fluid is removed from the uterus; 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).
AMNIOTIC FLUID:
Fluid in which the fetus floats in the uterus; fetal cells are found suspended in this fluid.
The gain or loss of chromosomal material ie. having missing or extra chromosomes (see Genetics Fact Sheets 6 and 7).
ANTIBODY:
A protein, produced in response to a foreign substance in the immune system.
ANTICIPATION:
The situation where a genetic condition appears to become more severe and/or
arise at an earlier age as it is passed through subsequent generations (seen in
many trinucleotide repeat
mutations).
ASSISTED REPRODUCTIVE TECHNOLOGY (ART):
A term describing a variety of measures employed to increase the possibility of pregnancy. Includes IVF (in vitro fertilisation).
ATP:
Stands for adenosine triphosphate. ATP is a chemical that is used in
cells to drive chemical reactions in which energy is produced.
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).
AUTOSOMAL DOMINANT
MUTATION:
A dominant mutation in a gene
which is carried on an autosome.
AUTOSOMAL GENE:
Any gene which is located on an autosome.
AUTOSOMAL RECESSIVE
MUTATION:
A recessive mutation in a gene
which is carried on an autosome.
AUTOSOME:
Any chromosome 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'.
BANDING:
A series of darkly and lightly stained stripes across a
chromosome are produced by treating the chromosomes with different
chemicals. There are a number of different staining techniques which produce
different patterns eg. G-banding, R-banding etc.
The sex chromatin or highly condensed inactive X chromosome present in each somatic cell in females (see Genetics Fact Sheet 14).
Two complementary basic building blocks (nucleotide bases) that make up the genetic code, combine to form one rung of the DNA ladder.
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).
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; characterised by generalised weakness and muscle wasting that affects limb and trunk muscles first; similar to Duchenne muscular dystrophy (DMD) 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 (teh inner cell mass).
Stem cells found in bone marrow that generate bone, cartilage, fat and fibrous connective tissue.
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).
Genes which act as a 'brake' on cell division and prevent uncontrolled cell division/proliferation.
CARCINOGEN:
A physical or chemical agent that causes cancer. It may or may not be a
mutagen.
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 cell contains two copies of each
gene. One gene copy may be mutated and
the other may be 'working'. 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 recessive to the other 'working' copy
of the gene. An individual who has one 'working' 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 mutant gene to their
offspring.
CARRIER SCREENING:
Testing populations to determine if individuals are carriers of a
mutated or faulty gene for a particular
condition.
CARRIER TESTING:
Testing an individual who is at risk due to a family history to determine if he
or she is a carrier of a mutated or faulty
gene for a particular condition.
CELL:
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.
CELL CULTURE:
A method for growing cells in the laboratory.
CELL DIVISION:
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 in the testis or
ovary is known as meiosis.
CENTROMERE:
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).
CHORION:
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 fetus 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).
CHROMOSOME:
A threadlike structure found in the nucleus of all
the body cells (except red blood cells) consisting of
DNA and proteins. Each chromosome can be
thought of as a string of beads where every bead represents a gene (see
Genetics Fact Sheet 1 ).
Please see 'Translocation'.
CLINICAL GENETICS:
A specialty of medicine concerned with the diagnosis and provision for risks of
developing an illness with a genetic basis in individuals and families.
Doctors with sub-specialty training in clinical genetics. Their role is in diagnosis, management and the provision of genetic counselling and appropriate genetic testing.
CLINICAL HETEROGENEITY:
Refers to the occurrence of clinically different types of genetic conditions due
to mutations in the same gene.
CLONE:
An identical copy of a gene/s or a group of identical
cells.
Please see 'Genetic cloning'.
Also known 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).
CODOMINANCE:
The equal expression of both copies of a gene in an
individual eg. presence of both haemoglobin A and S on electrophoresis in an
individual heterozygous 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.
DNA made in a laboratory from the messenger RNA (mRNA) expressed by a gene. The cDNA is made using an enzyme.
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.
The fusing (joining) of the sperm with the ovum, which leads to the development of an embryo.
CONCORDANCE:
Presence of the same characteristic in both members of a pair of twins (or set
of individuals). The opposite to discordant.
CONFINED PLACENTAL
MOSAICISM:
Mosaicism that is seen only in the placenta but
not in the fetus.
CONGENITAL:
Present at birth, not necessarily inherited.
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).
CONNECTIVE TISSUE:
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.
CONSANGUINITY:
Relationship between two individuals with a common ancestor, for example,
cousins (see Genetics Fact Sheet 16).
CONSULTAND:
The person seeking, or referred for, genetic
counselling.
CORDOCENTESIS:
Removal of a blood sample under ultrasound guidance
for prenatal diagnosis of certain conditions. The procedure is similar to
amniocentesis or trans-abdominal CVS (see Genetics Fact Sheet 17C).
CROSSING OVER:
When chromosome pairs join together during
meiosis (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.
CVS:
Please see 'Chorionic villus sampling'.
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).
CYTOGENETICS:
The microscopic study of chromosomes and how
changes in chromosome structure and number affect individuals.
One of the four basic building blocks (nucleotide bases) that makes up the genetic code (DNA). It is represented by the letter C.
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).
DELETION:
The loss of some genetic material from a chromosome.
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
DNA.
DEOXYRIBONUCLEIC ACID:
Please see 'DNA'.
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).
The identification and naming of an individual's disease or condition.
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.
The process cells undergo whereby they mature and develop the features of a specialised cell with a specific function such as a heart, liver or muscle cell.
Also called the gastrointestinal (GI) tract, is the multi-organ system that takes in food, extracts nutrients and expels waste.
DIPLOID NUMBER:
This is the number of chromosomes in the
somatic (body) cells. There are two copies of
each chromosome. In humans, the diploid number is 46.
DISCORDANT:
The situation where both members of a pair of twins do not exhibit the same
characteristics. Opposite to concordant.
DISOMY:
Meaning 'two bodies'. Where there are two copies of each chromosome. This is
the normal chromosome complement.
DIZYGOTIC TWINS:
Nonidentical twins, arising from two different eggs fertilised by two different
sperm; such twins are also referred to as fraternal
twins.
DNA
(DEOXYRIBONUCLEIC ACID):
The chemical compound that makes up genes within
chromosomes 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).
DNA SEQUENCING:
Determining the pattern or order in which the nucleotide
bases occur in a piece of DNA.
This sequence is the genetic code.
DOMINANT:
Every cell contains two copies of each gene. Where
only one of the gene copies or allele is mutated,
and the other allele is 'working', but the person is affected by a genetic condition due
to that mutation, the mutation is described as
dominant. The mutated gene is said to be dominant over the other 'working' 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.
The structural arrangement of DNA. The bases (basic chemical building blocks) pair up to form rungs on a ladder twisted into a helix, or coil.
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 the 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).
DUPLICATION:
A part of the chromosome 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 DNA structure
of the chromosome or a gene.
Refers to a blockage in the hollow tube that connects the stomach to the rest of the intestine.
DYSMORPHOLOGY:
Comes from the Greek DYS - meaning abnormal, disease, faulty, impaired and
MORPHOLOGY - meaning structure or form. Refers to the changes in the usual
structure of a person's cells, in particular external appearance.
DYSPLASIA:
Abnormal development, or growth, of tissues or cells.
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).
EGG:
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 method used to study DNA. It is the process of separating DNA molecules in a gel medium according to size and electrical charge, by applying an electric current.
EMBRYO:
The product of the fusion of an egg and a
sperm at conception. This term is reserved for the first eight weeks of
development.
Primitive (undifferentiated) cells from the embryo that have the potential to mature and develop into a wide variety of specialised cell types.
EMPIRIC RISK:
A risk estimate that is given for the chance of occurrence or recurrence of a
particular condition in an individual based on the observation of other
families with that condition.
ENVIRONMENTAL FACTORS:
Factors in the environment that may have an effect on our development or
growth eg. diet, atmospheric pollutants, cigarette smoke, preservatives, X-rays
etc.
ENZYME:
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 15).
ETHICS/ETHICAL
BEHAVIOUR:
Code of behaviour considered correct; especially that of a particular group,
profession or individual.
EUGENICS:
The practice of trying to influence human heredity
by encouraging the transmission of 'desirable' characteristics and discouraging
the transmission of 'undesirable' ones.
EXON:
The part of the DNA message
that is translated into a protein.
EXPRESSED GENE:
When the coded information contained in the gene is understood by the cells to
produce a product such as a protein.
EXPRESSIVITY:
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:
A characteristic or condition which tends to run in families.
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.
FAMILIAL HYPERCHOLESTEROLAEMIA (FH):
An inherited tendency to have high cholesterol, and this 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 gene that is not working or functioning properly.
FERTILISATION:
The joining of an egg and
sperm at conception to create an embryo.
FETAL BLOOD SAMPLING:
A prenatal diagnostic technique where a blood sample is obtained from the
fetus.
FETOSCOPY:
A prenatal diagnostic technique where the fetus and
the inside of the uterus (womb) can be directly
visualised.
FETUS:
In humans, the product of conception after the end of the eighth week of
pregnancy to the moment of birth.
Folate or folic acid is a form of water-soluble B group vitamins that is important in preventing neural tube defects (NTDs) in the developing baby during pregnancy.
FRAGILE SITE:
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).
A genetic conditon 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).
FRATERNAL TWINS:
Please see 'Dizygotic twins'.
GAMETE:
Refers to the sperm
cells in males and the egg cells in females.
G BANDS:
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.
GENE: The basic unit of heredity; a segment of DNA which contains the information for a specific characteristic or function.
GENE CLONING:
Isolating a gene and then making multiple copies of
it by inserting it into a bacterial cell or another
organism.
GENE MAPPING:
Determining the relative locations of different genes
on chromosomes.
GENE THERAPY:
Treating genetic conditions and diseases by replacing, manipulating or supplementing genes that are not working properly.
(see Genetics Fact Sheet 27).
Please see 'Carrier of a mutated gene'.
GENETIC CODE:
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).
GENETIC COUNSELLING:
The provision of diagnosis, information and support by health professionals with
specialised training in genetics and counselling (see Genetics Fact Sheet 3).
GENETIC COUNSELLOR:
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.
GENETIC ENGINEERING:
Laboratory techniques used to alter or manipulate the genetic make-up of cells
or an organism by deliberately removing, changing or inserting individual
genes.
GENETIC HETEROGENEITY:
Different mutations in a gene
causing the same condition.
GENETIC MAPPING:
Determination of the relative positions of genes on a
chromosome and a measure of the distance between them.
Having some genetic factor(s) that may make an individual more likely to develop a particular condition than the general population.
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).
The study of genes and inheritance patterns.
The complete set of genes carried by an individual or a cell.
Please see 'Imprinting' (see Genetics Fact Sheets 14 and 15).
The genetic constitution of an individual.
The cells of the body that are used in reproduction (egg and sperm).
The family of cells that divide to produce new germ cells.
When the germ cells (sperm or egg cells) have a different genetic make-up to the cells in the rest of the body.
A heritable change in the DNA that occurred in a germ cell (a cell destined to become an egg or in the sperm).
Please see 'Germline mosaicism'.
One of the four basic building blocks (nucleotide bases) that makes up the genetic code (DNA). It is represented by the letter G.
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).
This is the number of chromosomes in the sex cells (sperm or egg). There is one copy of each chromosome. In humans, the haploid number is 23.
A set of closely linked alleles on a chromosome that is normally inherited as a block.
Please see 'Deafness'.
The transfer of a gene from parent to child. In mothers, the gene is transferred via the DNA in the egg and in fathers the gene is transferred via the DNA of the sperm.
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).
The passing of genetic characteristics from parent to child.
The degree to which a characteristic is determined by genetics or genes.
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.
HOMOZYGOTE:
Refers to an individual in whom the two alleles or
gene copies contain identical information. An individual can be
homozygous for the working copies of the gene or can be homozygous for the
mutated copies of the gene.
HORMONE:
A chemical product of the body which has a specific regulatory effect upon the
cells.
The international scientific effort that began on 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).
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).
IDENTICAL
TWINS:
Twins that arise from a single egg fertilised by a single
sperm. These twins are therefore genetically identical. They are also
referred to as monozygotic twins.
IMMUNE SYSTEM:
The body's defence system that destroys potentially harmful, foreign
substances or organisms that enter the body. It is made up of specialised
cells.
The process whereby a fertilised egg (embryo) is embedded in the lining of a woman's uterus.
IMPRINTING:
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).
INCIDENCE:
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'.
INHERITED:
The transmission of genetic information from a parent to a child.
INSERTION:
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.
INTRON:
The part of the genetic sequence that is not translated into the final
gene product or message.
INVERSION:
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 causing the
genetic code to be read or translated incorrectly.
IN VITRO FERTILISATION (IVF):
The process whereby an egg is fertilised with
sperm in the test tube and then transplanted into a woman's
uterus.
ISOCHROMOSOME:
A chromosome in which the arms ('p'
and 'q') are of equal length and the information in
each of the two arms is genetically identical.
Please see 'in vitro fertilisation'.
KARYOTYPE:
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.
Kb:
A segment of DNA which is
1,000 base pairs in length.
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).
LINKAGE:
The tendency for genes or segments of
DNA which are located close together on the same
chromosome to be inherited together.
LOCUS:
The position on a chromosome of a segment of a gene.
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).
LYSOSOME:
A small body which is found in most cells, but is particularly frequent in the
cells of the liver and kidney. Lysosomes contain important enzymes.
MARKER CHROMOSOME:
A chromosome, or part of a chromosome, usually
small, of unknown origin.
MATERNAL SERUM TESTING:
A test which assesses the risk of fetal abnormalities
such as neural tube defects (NTDs) and
Down syndrome by analysing a number of biochemicals in the mother's
blood during pregnancy (see Genetics Fact Sheet 17B).
MEIOSIS:
The special cell division which only takes place in
the sex cells of females and males, resulting in egg and
sperm cells that contain 23 chromosomes
(the haploid number).
MENDELIAN INHERITANCE:
This refers to the inheritance of single genes and
follows specific patterns: autosomal
dominant, autosomal recessive
and X-linked inheritance (see Genetics Fact Sheets 8, 9 and 10).
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).
METABOLISM:
The physical and chemical processes by which energy is made available for
essential body functioning, growth and development.
METACENTRIC:
Refers to a chromosome which has its centromere
in the middle and the short ('p') and long ('q')
arms are of equal length.
MISCARRIAGE:
Loss of a baby before the twentieth week of pregnancy.
MITOCHONDRIA:
These structures or organelles in the cell are the
main energy source: they 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).
The genetic material contained in the circular genome found in mitochondria.
MITOSIS:
The process of cell division in all cells except the
reproductive cells. Mitosis results in 'daughter' cells which are genetically
identical to the parent cells.
MOLECULAR GENETICS:
The branch of genetics that studies the function and structure of
genes at the molecular level.
The smallest unit of a compound or substance that can exist by itself, and still have all of its chemical properties.
MONOCLONAL:
A group of cells that are all identical copies of an
original cell.
MONOGENIC:
A characteristic which is due to the information contained in a single
gene.
MONOSOMY:
Where one chromosome is represented once only
instead of twice; for example, girls with Turner syndrome have only one X
chromosome instead of the usual two copies (Monosomy X) (see Genetics Fact Sheet 32).
MONOZYGOTIC TWINS:
Please see 'Identical twins'.
MOSAIC:
A situation where some cells have an abnormal or
unusual genetic or chromosomal make-up while the rest of the cells in the body
have the usual genetic or 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.
mRNA:
An abbreviation for messenger RNA which
is the chemical that transfers the genetic
DNA message to the ribosomes where it is translated into
proteins.
MULTIFACTORIAL
INHERITANCE:
A pattern of inheritance which results from the interaction of one or more
genes with environmental factor(s), (see Genetics Fact Sheet 11).
MUTAGEN:
A physical or chemical agent that causes a permanent change (mutation) in a
gene. It may or may not be a carcinogen.
MUTATION:
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:
The embryonic structure which develops into the nervous system including the spinal cord and brain.
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).
The process of development of the brain and other parts of the central nervous system.
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 ususally 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).
Nerve cells, the structural and functional unit of the nervous system. A neuron transmits electrical signals around the body through a network of nerve cells.
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).
NONDISJUNCTION:
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).
A condition not related to a recognised pattern of characteristics and/or symptoms (syndrome).
NUCLEOTIDES:
Please see 'Bases'.
NUCLEUS:
The structure in a cell which contains the genetic
material.
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.
ONCOGENE:
A gene which, when triggered, can lead to cancer
(see proto-oncogene).
ORGANELLE:
Structures within cells such as the
nucleus, mitochondria and
lysosomes which have special functions.
OVUM:
The female reproductive cell or egg
which contains 23 chromosomes.
'P' ARM:
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.
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).
PEDIGREE:
A diagrammatic representation of a family health history tree.
PENETRANCE:
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. Such a genetic condition is said to have reduced or incomplete penetrance.
PHARMACOGENETICS/PHARMACOGENOMICS:
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).
PHENOTYPE:
The physical and/or biochemical characteristics of a person, an animal or other
organism which are determined by their genetic make-up and/or environment.
PLACENTA:
The structure that provides the fetus with
nourishment during development. It is attached to the wall of the
uterus and connects to the fetus through the umbilical cord.
Small circular DNA molecule used to transfer genes from one organism to another.
Cells that may still differentiate into various types of specialised tissue in the body.
POLYGENIC:
A condition or characteristic that is caused by many different
genes acting together.
POLYMORPHISMS:
Changed genes,
DNA sequences or chromosome structures
which occur naturally in the population and do not cause any harm to the
individual.
PREDICTIVE TESTING:
A form of genetic testing performed on a person with a family history of a particular genetic condition, but who does not have any synptoms of the condition at the time of testing. This testing determines whether 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).
PREDISPOSITION:
A situation in which a person, due to their inherited genetic make-up, may have
a particular susceptibility to a condition if exposed to the correct
environmental triggers.
PREIMPLANTATION GENETIC DIAGNOSIS (PGD):
An adjunct to the IVF process where the embryo undergoes genetic testing before it is transferred (implanted) into the uterus.
PRENATAL DIAGNOSIS:
The detection of fetal abnormalities during pregnancy (see Genetics Fact Sheets 17, 17A and 17C).
Tests during pregnancy to assess the possibility that a fetus is affected with a particular condition (see Genetics Fact Sheet 17B). These tests are not 100% accurate.
PRESYMPTOMATIC TESTING:
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 whether 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 in their life. Huntington disease is an example of a genetic condition where presymptomatic testing is used (see Genetics Fact Sheet 44).
PREVALENCE:
The proportion of a whole population affected by a certain condition.
PROBE:
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.
PROPHYLACTIC SURGERY:
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.
A small gland of the male reproductive system that secretes a fluid that makes up part of the ejaculate (seminal fluid).
PROTEIN:
Substances which are major components of the body structure, essential to body
function. They are made up of smaller units called amino
acids.
PROTO-ONCOGENE:
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.
'Q' ARM: RECESSIVE: RECURRENCE RISK: REGULATORY GENE: REPLICATION: RESTRICTION ENZYME: Small components in a cell that are composed of ribosomal RNA. They are important in the reading of the DNA messages in a cell. See also mRNA. 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): RIBONUCLEIC ACID: RING CHROMOSOME: RNA (RIBONUCLEIC ACID): ROBERTSONIAN
TRANSLOCATION: SATELLITES: The ability of a test to detect the presence of a genetic condition or a mutation when it is truly present. SEX CELLS: SEX CHROMOSOME: SEX INFLUENCED: SEX LIMITED: SEX LINKED: 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. 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 NUCLEOTIDE 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 exampe of a skeletal dysplasia is achondroplasia (see Genetics Fact Sheet 38). SOMATIC CELLS: SOMATIC GENE THERAPY: SOMATIC MUTATION: 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): SPORADIC 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 three mRNA sequences (UGA, UAG, UAA) that do not code for an amino acid and therefore signal that protein production or translation stops. STORAGE DISEASE: A gene in which if there is a change or mutation, this 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 that is not certain. SYNDROME: SYNTHESIS: An inherited degenerative condition of the nervous system where apparently healthy babies at about 6 months of age lose already acquired skills, gradually become blind, paralysed and unaware of his/her surroundings and die before approximately 5 years of age. It is more common amongst people of Ashkenazi Jewish background (see Genetics Fact Sheet 35). TELOMERE: TERATOGEN: TERMINATION OF
PREGNANCY: TETRAPLOIDY: TETRASOMY: An inherited blood condition in which the chains of the haemoglobin 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.
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. TRANSLOCATION: TRINUCLEOTIDE REPEAT: TRIPLET: TRIPLET
REPEAT (TRINUCLEOTIDE REPEAT): TRIPLOIDY: TRISOMY: Please see 'Patau syndrome'. Please see 'Edward syndrome'. Please see 'Down syndrome'. TUMOUR SUPPRESSOR GENE: Also called X0 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 chromosome (45,X), instead of the usual 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 kidney abnormalities (see Genetics Fact Sheet 32). ULTRASOUND: Please see 'Translocation'. A cell that has not matured into a functional, specialised cell type. UNIPARENTAL DISOMY: UTERUS (WOMB): 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). X-LINKED GENE: X-LINKED RECESSIVE
MUTATION: X-LINKED DOMINANT
MUTATION: ZYGOTE:
Each chromosome has two segments joined by the
centromere. The 'q' arm is the longer of these two segments. The
shorter segment is called the 'p' arm.
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 'working', 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 working 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).
The risk that an inherited condition will occur again in a family.
A gene containing information for the regulation
(switching on or off) of other genes.
The identical duplication of DNA
or of a cell.
Enzymes that can cut DNA into
strands at specific places along its length.
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.
An abbreviation for ribonucleic acid, a chemical similar to
DNA which has an important role in protein
manufacture. There are several types of RNA (see mRNA).
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.
The reproductive cells (sperm
or egg) which are the result of
meiosis.
An X or a Y chromosome which are different from the 22 autosomes.
A genetic condition or characteristic whose expression or severity differs between the
sexes. That is, it occurs more frequently in either males or females.
A genetic condition or characteristic that is expressed in only one sex. That is, it
occurs only in males or in females.
A genetic condition or characteristic which is determined by genes
carried on the X or Y chromosomes.
All the cells of the body except the reproductive cells (sex cells).
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.
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.
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 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.
The terminal or end segment of each chromosome
arm.
An substance that produces or increases the incidence of birth defects or
congenital abnormalities by interfering with development of the
fetus during 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.
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 is not
affected. An 'unbalanced' translocation results in the loss or gain of genetic
material which may result in a genetic disorder.
Please see 'Triplet Repeat'.
A sequence of three nucleotides in the
DNA sequence. Each triplet represents the code for a particular
amino acid.
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 to date 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.
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.
The use of sound waves for visualising body tissues and structures. In
pregnancy, structural abnormalities in the fetus can
be detected (see Genetics Fact Sheet 17A).
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 fetus develops into a baby over a period
of nine months.
Any gene which is located on the X
chromosome.
A recessive mutation
in a gene carried on the X chromosome.
A dominant mutation
in a gene carried 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 fetus.