Glossary of Genetic Terminology
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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).
CANCER PROTECTION GENES:
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 '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 recessive 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.
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).
CHROMOSOME TRANSLOCATION:
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.
CLINICAL GENETICIST:
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 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).
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.
COMPLEMENTARY DNA
(cDNA):
DNA made in a laboratory from
the messenger RNA (mRNA) expressed by a gene. The cDNA is made using an enzyme.
COMPLEX INHERITANCE:
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.
CONCEPTION:
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.
CULTURE MEDIUM:
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.