Understanding Rare Chromosome and Gene Disorders

Glossary

Unique GLOSSARY of genetic / genomic terms

General

Acrocentric a chromosome whose centromere is close to the end of the p-arm (chromosomes 13, 14, 15, 21 and 22).

Allele different versions of a gene.

Altered function a gene or protein that has changed so that it no longer functions as expected.

Amino acid the building blocks of proteins, coded for by genes. In the human body, there are 20 different amino acids that can be linked together to form a polypeptide. A protein may be made from one or more polypeptides. 

Aneuploidy the presence of a lower or higher number of chromosomes than expected. For example, we usually have 46 chromosomes, some people have 45 or 47 or more.

Autosomal dominant describes the way some genetic changes are or can be inherited. Autosomal means the change is in an autosome (chromosomes 1-22) and dominant means that features or symptoms can be observed when a single copy of the chromosome (or gene) is affected.

Autosomal recessive describes the way some genetic changes are or can be inherited. Autosomal means the change is in an autosome (chromosomes 1-22) and recessive means that features or symptoms can be observed only when both copies of the chromosome (or gene) are affected.

Autosome refers to any of the numbered chromosomes, 1-22.

Balanced translocation also known as a balanced reciprocal translocation, this is when two pieces of a chromosome (usually from different chromosomes) have ‘swapped places’, and no genetic material has been lost or gained.

Base part of DNA that holds two strands together and forms the DNA sequence. There are four bases: Adenine (A), Guanine (G), Cytosine (C) and Thymine (T).

Base pair DNA exists as a double stranded structure. The two strands of DNA are held together by bonds between base pairs. A always pairs with T and G always pairs with C.

Breakpoint(s) point(s) at which a chromosome has broken when a genetic change has occurred (such as a deletion, duplication, insertion, translocation).

Benign often used to describe a genetic change that is not expected to cause any features or symptoms of concern. Benign changes to DNA are common and nothing to worry about, not all genetics reports include this type of variation.

Candidate gene gene suspected of causing or contributing to features or symptoms.

Cell the basic building blocks of our bodies.

Centromere region of a chromosome between the p and q arms.

Chiasmata the places where chromosome pairs maintain contact so genetic material can be swapped (crossed over) during the formation of sperm and eggs. This results in a Unique set of chromosomes in each egg and sperm cell.

Crossing over each egg and sperm cell contains a single set of unique chromosomes made from the chromosome pairs of the parent. When these cells are formed, the parental chromosome pairs come together and swap genetic material, this is known as crossing over, they then separate as new chromosomes and one of each pair is placed in an egg or sperm. 

Compound heterozygote when both copies of a recessive gene are altered but in a different way. (Altering a single copy of a recessive gene does not result in features or symptoms, these are only observed when both copies are affected. If both copies of the same gene are altered in the same way, this is called homozygous).

Consanguinity being related to someone by blood rather than marriage, i.e. having the same ancestors. This increases the chances of having the same genetic change on both copies of a gene or chromosome, so recessive disorders are more common.

Consanguineous commonly used to describe a union between related couples, such as siblings, 1st and 2nd cousins or grandparent/parent/child. Children of consanguineous unions have an increased risk of having recessive gene disorders.

Chromosome long strand of densely packaged DNA located in the nucleus of cells.

Chromosome band each of our chromosomes has a unique and distinctive banding pattern visualised using a microscope following specific laboratory staining techniques. Bands are assigned a letter according to which arm they are on, and a number depending on where they are located on the chromosome arm (the resolution of bands depends on the chromosome preparation and staining technique used).

Chromosome disorder a change to the sequence and/or structure of a chromosome that causes features or symptoms.

Clinical geneticist also known as a medical geneticist, is a medical doctor who specialises in genetics. They provide genetic diagnoses, and relevant management recommendations and counselling services.

CNV copy number variant. This is a variation in the number of copies of a specific piece of DNA in a chromosome. For example, we normally have two copies of each autosomal chromosome (chromosomes 1-22); if a piece of DNA is missing from one chromosome but not the other, the new copy number for that section of chromosome is 1. If a piece of DNA is duplicated on one chromosome but not the other, the new copy number is 3. It is possible to have copy numbers of 0 or multiples of more than 3.

Deletion when a piece of DNA is missing. Deletions can vary in size from one base pair to tens, hundreds, thousands (kb) or millions (Mb) of base pairs. Those too small to be seen using a microscope are known as microdeletions.

Diploid when chromosomes are present as pairs. Most of our cells contain a diploid set of chromosomes, i.e. 23 pairs of chromosomes (46 chromosomes in total). Eggs and sperm however usually contain 23 single chromosomes, so when they join at conception, a total of 46 chromosomes is restored, as a set of 23 pairs.

Distal when referring to position in a chromosome, distal means far from the centromere.

DNA abbreviation of deoxyribonucleic acid, the chemical name for the molecule that carries our genetic information

Double Helix the structure formed by two strands of intertwined DNA

Duplication when a piece of DNA has been copied and is present as an extra piece (or multiple extra pieces). Duplications can vary in size from one base pair (bp) to tens, hundreds, thousands (kb) or millions (Mb) of base pairs. Those too small to be seen using a microscope are known as microduplications.

Epigenetic  modifications to genes that alter their activity (e.g. switch them on or off), and do not involve changes to the DNA sequence. 

Environmental factors features or symptoms are sometimes affected by environmental factors. Such factors can be medical, emotional and physical and include diet, exercise, hormones and exposure to toxins.

Exon DNA sequence (part of a gene) that codes for an amino acid (the building blocks of a protein).

Gain when extra genetic material is identified it is often referred to as a gain. This means there has been a duplication of a specific piece of DNA or chromosome. Gain is also used in a different way to describe ‘gain of function’ gene variants, which result in a protein being made that has a different function.

Gene Genes are the basic unit by which genetic information is passed from parent to child. They are made from stretched of DNA that carries coded information on how to make proteins (and RNA).

Genetic related to genes or DNA

Genetic counselling if somebody is offered a genetic test, it is important that they are also offered genetic counselling, to help explain all of the associated implications of genetic test results. This is usually performed by a genetic counsellor or clinical geneticist.

Genotype the genetic makeup of a cell or person, commonly used to describe genetic variations that contribute to a person’s observable traits.

Genome complete set of genetic material

Genome assembly this is also known as genome build and refers to the DNA sequence of the entire genome which can be used by researchers to map genes. Genome assemblies are constantly updated; each new major revision or ‘build’ has a new name e.g. hg19 (human genome 19) also known as GRCh37 [Genome Reference Consortium (GRC) human (h) genome assembly number 37 (GRCh37)]. The most recent genome build is called GRCh38 or hg38.

Genome build this is also known as genome assembly (see above).

Genomic related to the genome

Germline/Gonadal mosaicism this type of mosaicism means that a few egg or sperm cells carry a genetic change that is not identifiable by parental DNA tests using standard blood or cheek swab samples. The chance of having a pregnancy with the same genetic change are estimated at less than 1%.

GOF –an abbreviation of ‘gain of function’. GOF is usually used to describe a change in DNA sequence that results in a new protein being formed that has a new function or pattern of expression. This means the cell type and/or stage of development where the protein is usually active may be changed.

GRC Genome Reference Consortium. An international group of institutes that work together to improve the representation of reference genomes, including the human genome. 

Haploid when chromosomes are present as a single set (i.e. 23 chromosomes). Egg and sperm cells are haploid. Most of our body’s cells contain a diploid set of chromosomes, i.e. 23 pairs of chromosomes (46 chromosomes in total).

Haploinsufficiency when one copy of a gene (or number of genes) has been altered or lost leaving only a single functional copy of the gene(s). As a result, the gene product (most often a protein) is produced in insufficient quantities to maintain normal function.

Hemizygous when only one copy of a gene, piece of chromosome or entire chromosome is present, as opposed to two copies. Many genes on the X chromosome are hemizygous in boys and men since they usually only have one X chromosome.

Heterozygote a person with two different copies of a gene (or number of genes) (known as alleles).

Heterozygous when one copy of a gene (or number of genes) is different from the other copy, so that there are two ‘versions’ (known as alleles) of the same gene.

Homologous when referring to chromosomes, this means chromosomes belonging to the same pair, i.e. chromosomes 1-22, with one of each pair being inherited from each parent. When referring to the sex chromosomes (X and Y), where two X chromosomes are present, they would be referred to as a homologous pair, but where an X and a Y chromosome are present, they would not.

Homozygote a person with two copies of a gene (or number of genes) that are the same (known as alleles). 

Homozygous when both copies of a gene (or number of genes) are the same. This may mean that two unaffected genes are present or two identical gene variants are present.

Imbalanced translocation this is when two pieces of a chromosome (usually from different chromosomes) have ‘swapped places’, and some genetic material has been lost or gained.

Imprinted genes an epigenetic modification that causes genes to be expressed in a parent-of-origin-specific manner. If a gene has paternal imprinting it means the copy inherited from the father is ‘switched off’, while maternal imprinting ‘switches off’ the copy inherited from the mother. This happens as a natural event involving the same genes in everybody. This only has implications when imprinting is altered or imprinted genes are lost or duplicated.

Incidental findings when referring to genetic test results, these are previously undiagnosed genetic changes or unknown findings that are discovered unintentionally during a DNA analysis that are not thought to be associated with the features for which a person was tested. This is also known as secondary findings.

Insertional translocation when a segment of chromosome is inserted into a different chromosome or a different region in the same chromosome.

Interstitial when referring to copy number variants (CNVs) such as deletions and duplications, an interstitial CNV occurs when there is a loss or gain of genetic material within the chromosome, as opposed to right at the end of the chromosome.

Intron part of a gene that does not code for the amino acids that make a protein. Most protein-coding genes have more than one exon (the amino acid-coding part) separated by introns.

Inversion a chromosomal inversion is a rearrangement of genetic material, where a piece of chromosome has broken off and has reinserted ‘back to front’. The segment of chromosome will be in the same place on the chromosome unless it has ‘translocated’ to a different place on the same chromosome or to a different chromosome. There are two types of inversion based on whether the centromere is involved; paracentric or pericentric.

ISCN International System for Human Cytogenetic Nomenclature, is the international standard used for naming, abbreviating and symbolising human chromosomes and genetic changes.

Isochromosome an unusual chromosome that forms when two short (p) or two long (q) arms of the same chromosome join at a centromere and are arranged as a ‘mirror image’ of each other. Isochromosome formation involves a chromosome duplication and deletion. The extra arm in the new chromosome is a duplication and the arm that is consequently lost is a deletion.

Isodicentric chromosome an isochromosome that has two centromeres.

Karyotype an individual’s set of chromosomes, often visualised by a specific laboratory staining technique. Visualising chromosomes in this way allows for identification of changes to chromosome number and structure (e.g. translocations and large deletions and duplications).

LCR abbreviation of ‘low copy repeats’. LCRs occur where short stretches of DNA are repeated at close intervals. The presence of LCRs can cause alignment problems and result in the deletion(s) and/or duplication(s) of chromosome segments. [More technically, this is known as non-allelic homologous recombination (NAHR)]

LOF abbreviation of ‘loss of function’. LOF is commonly used to describe a genetic change (variant) that causes a gene product (usually a protein) to be altered to such an extent that it no longer functions, or functions at a much reduced level.

LOH an abbreviation of ‘loss of heterozygosity’. LOH is used to describe the presence of a piece of chromosome from only one parent. LOH can occur if a piece of chromosome (or an entire chromosome) from the other parent is missing (a deletion) or if both copies originate from the same parent.

Loss when genetic material is missing, it is often referred to as a loss. This means there has been a deletion of a specific piece of DNA or loss of a chromosome. Loss is also used in different ways to describe ‘loss of function (LOF)’ gene variants, and ‘loss of heterozygosity (LOH)’.

Microdeletion a small deletion of DNA, usually less than 500,000 base pairs and not visible by chromosome staining (see deletion for further details).

Microduplication a small duplication of DNA, usually less than 500,000 base pairs and not visible by chromosome staining (see deletion for further details).

Missense describes a gene variant that results in a protein-coding sequence being altered so the protein is no longer made as expected. It usually involves one or more amino acid, the building block of a protein, being changed. While this may have no effect on the protein, sometimes the function of the protein is altered or lost.

Molecule the smallest unit of a chemical element or compound.

Monosomy when an entire chromosome is missing. When a part of a chromosome is missing it is known as partial monosomy. Monosomy is a form of aneuploidy. 

Mosaicism when referring to genetic changes, this means that the change is not found in all cells. There are two types of genetic mosaicism, germline/gonadal mosaicism and somatic mosaicism.

Mutation in genetics, a mutation is a change in a DNA sequence, this is more commonly known as a ‘variant’.

Nonsense describes a gene variant that results in a protein-coding sequence being altered so production of the protein stops prematurely. This results in a shorter (truncated) protein being made that either does not function or has an altered function.

Nucleotide the basic building block of DNA (and RNA). A DNA sequence is a long chain of nucleotides each consisting of a sugar molecule, a phosphate group and a base (G, A, T or C).

Nucleus a structure inside our cells that contains our chromosomes.

p-arm the small arm of a chromosome.

Paracentric inversion this type of chromosomal inversion occurs when a segment from one arm of a chromosome, that does not include the centromere, has broken off and reinserted itself ‘back to front’.

Pathogenic this word means ‘disease causing’. When it’s used to describe a genetic change it means that features or symptoms have been observed in association with the change.

Penetrance refers to the proportion of people with a particular genetic change e.g. a duplication, deletion or gene variant, who exhibit the features and symptoms of a genetic disorder. If some people with the genetic change don’t develop the features associated with the disorder, the condition is said to have incomplete (or reduced) penetrance. This can make it extremely challenging to provide accurate genetic counselling, since genetics professionals are unable to accurately predict how each person will be affected.

Pericentric inversion this type of chromosomal inversion occurs when a segment of chromosome including the centromere has broken off and reinserted itself ‘back to front’, and means the chromosome has a break in both arms.

Phenotype a person’s observable features for example physical appearance, development and behaviours. Phenotypes are a result of a person’s unique genetic make-up as well as environmental factors.

Polygenic risk score (PRS) this is a value given to express someone’s likelihood of having or developing a medical condition based on multiple variations to their DNA sequence. Some genetic variants may have a very small effect on their own, but the presence of a number of genetic variants acting together can influence the risk that health and development are affected.

Polyploidy this is when one or more extra chromosomes are present. For example, an extra X chromosome. It is incredibly rare to have an extra autosome.

Protein molecules made from one or more chains of amino acids (polypeptides) coded for by gene sequences. Proteins play many important roles in our bodies including structural, mechanical, biochemical and signalling.

Proximal if a genetic change occurs close to the centromere it is called proximal.

q-arm the long arm of a chromosome.

Regions of repetitive DNA The DNA from which our chromosomes are made contain many regions of different types of repeated sequence. They are a potential source of genetic variation that may have complex roles that affect gene activity. They are also regions where DNA anomalies are more likely to occur.

Robertsonian Translocation a translocation that commonly involves acrocentric chromosomes (chromosomes 13, 14, 15, 21 and 22). Two chromosomes (either from the same pair or different pairs) break at the centromere and the q arms join to form a larger chromosome with a single centromere. The p arms join to form a small chromosome which is often then lost; if this chromosome is lost, the person will have 45 chromosomes.

Ring a ring chromosome is formed when the tips of a chromosome are lost and the broken ends fuse together to form a ring-like structure.

ROH is an abbreviation of runs of homozygosity. This refers to stretches of DNA that are the same on both chromosome copies, and can be a sign of UPD or parents being related.

Secondary findings see incidental findings.

Sequence in genetics this commonly refers to a DNA sequence, normally one that codes for a gene. It will be a string of letters including only A, C, G and T (nucleotide bases). Proteins can also have a sequence based on their amino acid content.

Sex chromosomes chromosomes X and Y are sex chromosomes and are involved in establishing biological sex. Males usually have an X and a Y chromosome, females usually have two X chromosomes.

Skewed X inactivation when the natural, and usually considered random, process of X-inactivation appears to be preferentially inactivating a specific X chromosome, i.e. does not appear to be random.

SNP single nucleotide polymorphism. This is the substitution, in some people, of a single nucleotide (the building block of DNA) at a specific position in the genome. For example, the majority of people may have a nucleotide with the base G at a certain position on a chromosome, whereas some people may have an A.

SNV single nucleotide variant. This is the substitution of a single nucleotide (the building block of DNA) at a specific position in the genome that is not commonly found in the general population and is thought to be disease causing. For example, the majority of people may have a nucleotide with the base G at a certain position on a chromosome, but a few people may have a T, and as a consequence may show altered health and development.

Somatic mosaicism when some cells in the body contain a genetic variant and some do not (somatic cells are the cells that form our body). This happens when the variant occurred during fetal development rather than within an egg or sperm.

Splice site variant this is a gene sequence variant that affects the way a protein is made. Protein coding gene sequences contain stretches of DNA called introns and exons. Introns are ‘spliced out’ of the sequence so only exons are used to make a protein. A change to a splice site can disrupt the removal of an intron sequence.

sSMC a small supernumerary marker chromosome is a small additional duplicated piece of a chromosome that exists alongside the usual set of chromosome e.g. an isodicentric, ring or minute chromosome.

Syndrome when a group of features or symptoms are consistently identified in people with the same genetic change, it can be known as a syndrome.

Telomere these are DNA sequences at the ends of chromosomes made from repetitive non-coding DNA which protect the chromosomes. When a cell divides, the telomeres become shorter; when they become too short the cell stops dividing.

Terminal deletion/duplication this is when a chromosome deletion or duplication is located at the end of a chromosome arm.

Tetrasomy this is when a particular chromosome (or part of a chromosome) is present as four copies instead of the usual two, e.g. XXXX instead of XX. 

Translocation this is when a piece of chromosome has moved from its usual position. There are different types of translocations e.g. insertional, balanced, unbalanced, Robertsonian, X:autosomal.

Triploidy this term is used by medical professionals to describe children with an extra set of chromosomes, so they have 69 chromosomes instead of 46.

Trisomy this is when a particular chromosome (or part of a chromosome) is present as three copies instead of the usual two, e.g. XXX instead of XX. 

Unbalanced translocation when two pieces of a chromosome (usually from different chromosomes) have ‘swapped places’, but in doing so, some genetic material has been lost or gained.

UPD an abbreviation of uniparental disomy, when both chromosomes (or part of a chromosome) are inherited from the same parent instead of having one copy from each parent.

Variable expressivity is a term used to describe the range and severity of features that occur in different people with the same genetic condition. Some people may show no features or symptoms or be only very mildly affected; others may be more severely affected. A genetic condition with variable expressivity can make it extremely challenging to provide accurate genetic counselling, since genetics professionals are unable to accurately predict how each individual will be affected.

Variant also known as a mutation. This is a change in DNA sequence.

VUS an abbreviation of ‘variant of uncertain/unknown (clinical) significance’. VUS is used to describe a genetic variation that has been identified in someone with unexplained features or symptoms but it is uncertain whether they are associated.

X:autosomal translocation when a piece of chromosome has moved from its usual position on an autosome (chromosomes 1-22) to an X chromosome, or when a piece of the X chromosome has moved to an autosome. The translocations can be balanced (when no genetic material is lost or duplicated) or unbalanced (when genetic material is also lost and/or duplicated). This type of translocation has the added complication of possible gene silencing associated with X-inactivation.

X-inactivation X-inactivation is a natural process that inactivates all but one X chromosome in our cells. One of the X chromosomes in XX females is inactivated in each cell early in development. Selection of which X chromosome is inactivated is assumed to be random but this is not always the case. Skewed X-inactivation occurs when one X chromosome is inactivated significantly more often than the other. The X chromosome of XY males is not inactivated since there is only one X chromosome present in each cell. People with additional X chromosomes, are expected to have only one fully active X chromosome in each cell. (Unique publishes a guide to X-inactivation).

X-linked relating to the X chromosome. X-linked may refer to a gene(s), a section of the chromosome or a variant.

Genetic tests

Amnio Amniocentesis. This is a when a sample of the fluid that surrounds an unborn baby during pregnancy (amniotic fluid) is taken for analysis. The fluid contains cells that can be used for genetic testing. An amniocentesis can be performed after week 15 of pregnancy.

Array CGH array comparative genomic hybridisation. This is a genetic test/technology that compares a sample of DNA to a ‘standard’ set of DNA sequences. This technique can be used to identify chromosome deletions and duplications as well as uniparental disomy/loss of heterozygosity.

CMA chromosome microarray analysis. CMA is a high resolution genetic test that analyses a person’s entire genome using microchip based technology. There are different types of CMA, e.g. Array CGH and SNP array.

CVS chorionic villus sampling. This is a when a sample of cells is taken from the placenta for analysis during pregnancy. Chromosomes/DNA are then isolated and used for genetic testing. A CVS is usually performed between weeks 11 to 14 of pregnancy.

ES exome sequencing. This is when only the DNA sequence of exomes is analysed as opposed to sequencing the entire genome. Exomes are the parts of genes that code for proteins.

FISH fluorescence in situ hybridisation. This is a technique used to look at chromosome changes such as deletions, duplications and translocations. Pieces of DNA are labelled with a fluorescent marker and stuck to sample chromosomes, changes are visualised by microscopic analysis of fluorescently labelled chromosomes.

G Banding staining (Giemsa or G) of condensed chromosomes to show light and dark banding patterns.

Gene specific panels microarray panels that represent specific genes as opposed to the entire genome.

Ideogram schematic representation of a chromosome banding pattern

Methylation the addition of a methyl group. DNA methylation tests are used to assess the activity of genes (since methylation of DNA at the beginning of a gene typically reduces its activity). These tests can be used when looking into gene imprinting and X inactivation.

MLPA multiplex ligation-dependent probe amplification. This test has many uses, such as duplication/deletion/sequence variant detection as well as DNA methylation analysis. Probes are used that recognise specific DNA sequences and make multiple copies of the sequence (if it’s not been deleted), and the results can be quantitated using different technologies. 

NGS next generation sequencing. NGS is used to describe a number of modern, high throughput and low cost DNA sequencing technologies.

NIPD non-invasive prenatal diagnosis. NIPD is a technique used to screen for genetic disorders in an unborn fetus using a blood sample from the pregnant mother. The test analyses the tiny amount of fetal (placental) DNA that can be found in a mother’s bloodstream and therefore reduces the risks associated with invasive testing. NIPD is more commonly used to test for specific gene variants but is currently only available for a small number of genes.

NIPT non-invasive prenatal testing. NIPT is a technique used to screen for genetic disorders of an unborn fetus using a blood sample from the pregnant mother. The test analyses the tiny amount of fetal (placental) DNA that can be found in a mother’s bloodstream and therefore reduces the risks associated with invasive testing. NIPT is more commonly used to test for changes in number of whole chromosomes, it is not routinely used or available for other chromosome disorders.

PCR polymerase chain reaction. This is a laboratory technique that generates multiple copies of a DNA sequence using sequence-specific probes and an enzyme called polymerase. It is the basis of many other techniques used in genetic testing, such as MLPA. 

PGD preimplantation genetic diagnosis (now known as PGT-M). PGD is used to analyse DNA from tissue samples taken from embryos that form during IVF treatment. It is used to detect common or suspected single gene disorders. PGD is a technique employed to help families affected by a serious genetic condition to avoid passing it on.

PGS preimplantation genetic screening (now known as PGT-A). This test is used to identify changes in the number of whole chromosomes.

PGT pre-implantation genetic testing. PGT is used to analyse DNA from tissue samples taken from embryos that form during IVF treatment. It is used to detect common or suspected genetic disorders. There are different types of PGT.

PGT-A preimplantation genetic testing for aneuploidies. This test is used to identify changes in the number of whole chromosomes.

PGT-SR Preimplantation Genetic Testing for chromosome Structural Rearrangements. This test is used to identify changes to the structure of chromosomes such as inversions or translocations which can also result in the loss or gain of duplicated genetic material.

PGT-M Preimplantation Genetic Testing for Monogenic single gene disorders. PGT-M is used to analyse DNA from tissue samples taken from embryos that form during IVF treatment. It is used to detect common or suspected single gene disorders. PGD is a technique employed to help families affected by a serious genetic condition to avoid passing it on.

SNP array single nucleotide polymorphism array. A type of DNA microarray that detects single base variations to DNA sequences.

Targeted genomic testing when specific genetic changes are investigated.

Trio analysis when the DNA of a child is analysed together with the DNA from both parents.

WES whole exome sequencing. This is when only the exons of genes (the parts that code for proteins) are sequenced (i.e. the sequence of letters GATC of all or a subset of genes. This is about 1% of the human genome).

WGS whole genome sequencing. This is when the entire genome (complete set of genetic material) is sequenced.

Genetic tests results

A abbreviation for Adenine, one of the four DNA bases (GATC)

add additional material of unknown origin

arr array

bp base pair

c cDNA

C abbreviation for Cytosine, one of the four DNA bases (GATC)

cen centromere

cgh comparative genomic hybridisation

codon set of 3 nucleotides that form a unit of genetic code

del deletion

de novo latin terminology meaning ‘a new event’

der derivative

dic dicentric

dn abbreviation of de novo

fs frameshift

dup duplication

G abbreviation for Guanine, one of the four DNA bases (GATC)

GRCh37 Genome Reference Consortium human 37

h human

hg human genome build

hg18 human genome build number 18

hg19 human genome build number 19

i isochromosome

idic isodicentric

ins insertion

inv inversion

ISCN International System for Human Cytogenetic Nomenclature

ish in situ hybridisation

kb kilo bases

mar marker chromosome

mat maternal

mos mosaic

Mb megabases

P protein

p short arm of a chromosome

pat paternal

psu pseudo

q long arm of a chromosome

r ring chromosome 

rcp reciprocal 

rea rearrangement 

rec recombinant chromosome

rob robertsonian translocation

s satellite

subtel subtelemeric, positioned before the end of the chromosome

T abbreviation for Thymine, one of the four DNA bases (GATC)

t translocation 

tel telomere 

ter terminal (end of chromosome) 

trp triplication 

upd uniparental disomy 

var variant or variable region 

wcp whole chromosome paint

* a ‘stop codon’ is introduced and protein translation is stopped at this point

x (multiplication sign) Multiple copies of rearranged chromosomes or number of copies of a chromosomal region

(minus sign) Loss 

+ (plus sign) Gain 

> (arrow) from – to 

(  ) (brackets/parentheses) Surround structurally altered chromosomes and breakpoints

[  ] (square brackets) surround number of cells

: (single colon) chromosomal break

; (semicolon) separates altered chromosomes and breakpoints in structural rearrangements involving more than one chromosome

:: (double colon) chromosomal break and reunion

, (comma) separates chromosome numbers, sex chromosomes and chromosome changes

. (decimal point) denotes sub-bands

. (full stop/period) Separates analytical techniques 

? (question mark) Questionable identification of a chromosome or chromosome structure 

/ (slanted line) separates cell lines (used in mosaic karyotypes)