AGSA Glossary of Terms 

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

[A]

Acrocentric - 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. 

Allele - one member of a pair of genes at a given region of DNA. 

Amyloid - a complex material composed partly of proteins which is present in the body of people suffering from several diseases. 

Anticipation - the occurrence of a genetic disorder at earlier age and/or at greater severity in successive generations. 

Autosomal Dominant - determined by a gene on one of the chromosomes other than the sex chromosomes, requiring only one member of the gene pair to be altered to express the condition. 

Autosomes - the 22 pairs of chromosomes not including the two sex chromosomes. 

[B]

Band - 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. 

[C]

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 or "pinched in" part of the chromosomes separating the short (p) arm from the long (q) arm. 

Chromosome arm - see centromere. 

Chromosome number - in humans, the chromosome number is 23 pairs giving a total of 46.  Half of the chromosomes are inherited from the mother and half from the father. 

Chromosome - chromosomes contain genetic material and are in every cell of the body. 

Chromosome - 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 the pair is inherited from the mother, the other from the father. 

Cytogenetics - the study of the number, structure, behaviour and effects of chromosomes. 

[D]

Deletion - 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 of the end of a chromosome; interstitial deletion is the loss of material from within the chromosome, between the ends. 

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

Disomy - meaning two bodies.  Refers to a normal chromosome pair. 

DNA (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. 

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

[F]

Factor IX - blood coagulation factor. 

F.I.S.H. - stands for Fluorescence In-Situ Hybridisation - the technique whereby small pieces of DNA (probes) which "recognise" particular chromosomes or parts of chromosomes are labelled with a coloured fluorescent dye and then used to highlight "target" DNA in chromosomes. 

[G]

G6PD - the important red-cell enzyme defect, glucose 6-phosphate dehydrogenase deficiency, is particularly prevalent in parts of the Middle East, Mediterranean, South East Asia and in people of African decent. 

Gamete - the germ cell (sperm or ovum) continuing a single haploid set of (23) chromosomes from one parent. 

G-band - see band.  The common banding produced with Giemsa stain and used to routinely identify chromosomes. 

Gene - a sequence of genetic information (DNA code) which is responsible for a particular bodily function or characteristic. 

[H]

Haploid - the chromosome number (23) found in the parental gamete. i.e. half the diploid number. 

Heterozygous - an individual with two different alleles of a gene region. 

Hypercholesterolaemia - a condition which leads to elevated levels of plasma cholesterol. 

[I]

Imprinting - where gene expression is influenced by the parental origin.

Imprinting - 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. 

Interstitial - any part of a chromosome between the ends (telomeres).  See deletion. 

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

ISCN - the International System of Cytogenetic Nomenclature used to describe chromosomal constitutions. 

[L]

Linkage - refers to genetic factors which are inherited together, because they are closely located to one another. 

[M]

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

Melosis - the process of cell division leading to formation of eggs and sperm with halving of the chromosome number. 

Mendelian - following the patterns of inheritance proposed originally by Gregor Mendel. 

Metacentric - a metacentric chromosome has its centromere in the middle and the p and q arms are of approximately equal length. 

Meiosis - see cell division. 

Mitosis - see cell division. 

Modal number - the chromosome count characteristic of cells from a particular individual (usually 46 in a normal human). 

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

Mosaicism - 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. 

Multifactorial - determined by multiple genes and also by non-genetic factors. 

[P]

Phenotype - the visible expression of the action of a particular gene; the clinical picture resulting from a genetic disorder. 

Polygenic - determined by multiple genes. 

Polymorphism - 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. 

Probe - a small fragment of DNA designed to attach to 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.

[R]

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 partial trisomy or monosomy, the translocation is said to be unbalanced. 

Ring chromosome - a chromosome which has broken at each end, lost the terminal fragments and curled around to join at the ends to form a ring.  Formation of a ring results in the loss or deletion of the terminal segments. 

Robertsonian translocation - a type of translocation exclusive to the acrocentric chromosomes, 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 translocations. 

[S]
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. 

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

[T]

Tandem Repeat Sequence - a series of identical DNA sequence lying adjacent to each other in the same orientation, within a much larger DNA sequence. 

Telomere - the terminal segment of each chromosome arm. 

Tetrasomy - the presence of four, instead of two, of a particular chromosome. 

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

Translocation - 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 particular chromosome segments, it is said to be unbalanced. 

Triploidy - the presence in a cell of three haplid sets (23) of parental chromosomes, giving a total chromosome count of 69. 

Trisomy - the presence of three copies of a chromosome, i.e. trisomy 21.  Three copies of part of a chromosome is a partial trisomy. 

[U]
Uniparental disomy - where both members of a chromosome pair are contributed by the one parent rather than one from each parent. 

[X]
X-chromosome - see sex chromosome. 

[Y]
Y - chromosome - see sex chromosome. 

[Z]
Zygote - the single cell with 46 chromosomes resulting from the fertilisation of an ovum (23) by a sperm (23).  Through cell division (mitosis), the zygote develops into a multicellular embryo and then into the foetus.