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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 detailed information should be sought from qualified practitioners.


Acrocentric chromosomes are those with the centromere very close to one end, giving the chromosome a “V” shaped appearance. The short (p) arms are very short and usually have small dot-like appendages on stalks, known as “satellites”. The acrocentric chromosomes are 13,14,15,21 & 22.


The 22 pairs of chromosomes not including the two sex chromosomes.


The series of darkly and lightly stained stripes across a chromosome produced by a variety of chemical treatments. Each chromosome has its own characteristic banding pattern which allows specific identification of chromosomes or parts of chromosomes.


Involving or inherited from both parents 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.


The constricted or “pinched in” part of the chromosome separating the short (p) arm from the long (q) arm. It is the attachment point for the mitotic spindle during cell division.

Centromeric Fission

The constricted or “pinched in” part of the chromosome separating the short (p) arm from the long (q) arm. It is the attachment point for the mitotic spindle during cell division.


A special kind of mosaicism in which an individual or tissue contains a mix of cells derived from two genetically different individuals. The blood of a patient who had a transfusion from another individual would be chimeric.


Thread-like structures consisting of DNA which “package” genetic information within the nucleus of most cells of the body. Human cells contain 46 chromosomes in 23 pairs. The 46 chromosomes contain all the genetic information inherited from both parents, thus one of each pair is inherited from the mother, the other is from the father. The ability to see chromosomes through a microscope led to the field of science known as cytogenetics.

Chromosome Arm

See Centromere Chromosome Number: In humans, the chromosome number is 23 pairs giving a total of 46. Half of the chromosomes (23) are inherited from the mother and half from the father.


Loss of a whole chromosome or part of a chromosome. The term usually refers to loss of part of a chromosome (partial deletion). A terminal deletion refers to breakage and loss off the end of a chromosome; interstitial deletion is the loss of material from within the chromosome, between the ends.


A cell with a normal chromosome number including two sets of parental chromosomes (23 from each parent). See triploidy and tetraploidy.


Meaning two bodies. Refers to a normal chromosome pair.


Deoxyribonucleic Acid. The long double-stranded chemical molecule found in the nucleus of cells and packaged into chromosomes. DNA contains the “genetic code” (genes) which we inherit from our parents.


A double copy of part of a chromosome resulting in an extra (abnormal) dose of the duplicated material.


That part of the chromosomal DNA which is genetically active and pale staining with G- and C- banding. Euchromatin is relatively rich in GC base pairs. During interphase it is uncoiled and transcriptionally active whereas heterochromatin is condensed and inactive.


Something which has a tendency to occur within a particular family. For instance, Down syndrome which has occurred more than once within a family because of an inherited chromosome translocation, can be said to be familial in that family.

Fetal Blood Sampling

A prenatal diagnosis technique where a blood sample is obtained from the fetus.


Stands for Fluorescence In-Situ Hybridisation. The technique whereby small lengths of DNA (probes) which “recognise” complementary DNA on particular chromosomes or parts of chromosomes, are labelled with a coloured fluorescent dye and then used to highlight “target” DNA in chromosomes.

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.


Banding patterns on chromosomes make it easier to examine the chromosomes under the microscope for abnormalities in structure and/or number. The common banding produced with Giemsa stain and used to routinely identify chromosomes.


The germ cell (sperm or ovum) containing a single set of (23) chromosomes from one parent.


A sequence of DNA within a chromosome, containing genetic information (genetic code) which is responsible for a particular bodily function or characteristic. Genes are not visible through a microscope.

Gene Cloning

Isolating a gene and then making multiple copies of it by inserting it into a bacterial cell or another organism.

Gene Therapy

A method of treating genetic conditions by inserting a correct copy of the gene in question into the cells of individuals who have a mutated gene.


The scientific study of characteristics which are passed on (inherited) through successive generations in humans and other organisms due to the transmission of genetic material (DNA).


Refers to the sum total of genetic information contained in the chromosomes from a cell representing an individual person. A haploid genome would be the information representing the chromosomes from one gamete.


The chromosome number (23) found in the parental gamete, i.e. half the diploid number.


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 on the DNA of the sperm.


Chromosomal material which is condensed and genetically inactive during interphase. It consists of repetitive DNA sequences which are relatively rich in AT base pairs and is late replicating in the cell cycle. In metaphase chromosomes it is dark staining with G- and C- banding (see ‘euchromatin’).


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.


A chemical product of the body that has a specific regulatory effect upon the cells.


The ‘memory’ held by a chromosome as to which parent it was inherited from. The memory is chemically “stamped” into the DNA and can result in chromosomes behaving differently, depending on the parent of origin.


Any part of a chromosome between the ends (telomeres). See ‘deletion’.


The breakage of a chromosome in two places followed by a 180 degree switch of the segment between the breaks. There is no loss or gain of material but the sequence of the inverted segment is reversed.


The International System of Cytogenetic Nomenclature provides the terminology used to describe chromosome constitutions.


A metacentric chromosome in which the p and q arms are of equal length and the information in each of the two arms is genetically identical (iso = equal).


Usually refers to the cut out and arranged pairs of chromosomes from a photograph of a cell representing an individual. The term can also refer the the chromosomal constitution of a person, i.e “she has a normal female karyotype”.


A chromosome, usually small, of unknown origin and unidentifiable from its G-banding pattern.


See ‘cell division’.


A metacentric chromosome has its centromere in the middle and the p and q arms are of equal length.


See ‘cell division’.

Modal Number

The chromosome count which is characteristic of cells from a particular individual or cell line (usually 46 in a normal human).


A single copy of a chromosome rather than a pair (disomy).


The co-existence of genetically different cells within the same individual or tissue. e.g. a mixture of both normal cells and cells with a chromosome abnormality.


The failure of homologous chromosomes to segregate into each daughter cell during mitotic or meiotic anaphase. i.e. both members of a pair move to the same daughter cell, resulting in deficiency in one cell and excess in the other.


Pharmacogenomics is the study of how an individual’s genetic makeup affects the body’s
response to particular medicines (The Pharmacy Guild of Australia)


Literally many shapes. Variations of chromosome appearance which are considered to be normal. These ‘normal variants’ are passed on within families without causing any problems in development.


A small segment of DNA of known origin, designed to recognise complementary DNA on specific parts of chromosomes. The probe can be labelled with a coloured stain and used to confirm the presence or absence of the target material or to identify unknown chromosomal material.

Reciprocal Translocation

The breakage and exchange of material between two chromosomes without any gain or loss of genetic material. Such a reciprocal translocation is said to be balanced. Where translocated chromosomes are passed on to offspring in a way which results in gain or loss of material, the translocation is said to be unbalanced.

Ring Chromosome

A chromosome which has broken at each end, lost the terminal segments, curled around and rejoined to form a ring shape. Formation of a ring may result in the loss or deletion of the terminal segments.

Robertsonian Translocation

A type of translocation exclusive to the acrocentric chromosomes (13,14,15,21,22), in which two chromosomes join at or near their centromeres. This is effectively a fusion between two whole chromosomes rather than an exchange of material as in reciprocal translocation.


Small round appendages attached by fine stalks to the ends of the short (p) arms of the acrocentric chromosomes (13,14,15,21,22). They do not always stain darkly and may be difficult to see. Satellites on different chromosomes are often attracted to one another, causing the acrocentric chromosomes to be in satellite association.

Sex Chromosome

The X and Y chromosomes which are responsible for sexual development. In humans, females have two X chromosomes while males have an X and a Y. Genes on the Y chromosome are responsible for male sexual development.


Chromosomes with centromeres which are not centrally placed. The chromosome arms are not of equal length. This term does not apply to the acrocentric chromosomes which have an almost terminal centromere.


The particular collection of physical features or characteristics in an individual which help to identify or characterise a genetic abnormality. A group of symptoms, occurring together, which characterise a disease.


The terminal segment of each chromosome arm.


The presence in a cell of four haploid sets (23) of parental chromosomes, giving a total chromosome count of 92.


The presence of four, instead of two, of a particular chromosome.


The fusion or exchange of material between chromosomes. When there is no gain or loss of material, the translocation is said to be balanced; when there is gain or loss, resulting in trisomy or monosomy for a particular chromosome segment, it is said to be unbalanced.


The presence in a cell of three haploid sets (23) of parental chromosomes, giving a total chromosoome count of 69.


The presence of three copies of a particular chromosome, i.e. trisomy 21. Three copies of part of a chromosome is partial trisomy.

Uniparental Disomy

Both members of a chromosome pair are contributed by one parent rather than one from each parent.


See sex chromosome.

X-Linked Gene

Any gene which is located on the X chromosome.

X-Linked Recessive Mutation

A recessive mutation in a gene carried on the X chromosome.


See sex chromosome.


The single cell with 46 chromosomes resulting from the fertilization of an ovum (23) by a sperm (23). Through cell division (mitosis), the zygote develops into a multicellular embryo and then into a fetus.

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