Glossary

Adenine
One of the nucleotide bases in DNA and RNA.

Alleles
Variant forms of the same gene.

Allelic heterogeneity
The same genetic disease caused by different mutations in the same gene

Amino acid
The basic unit from which proteins are made.

Aneuploid
Having an abnormal chromosome complement, for example with missing or extra chromosomes

Antenatal testing
Testing a fetus before birth.

Autosomes
The chromosomes that are not concerned with sex determination. Humans have 22 pairs of autosomes, plus two sex chromosomes.

Base
An alternative term for nucleotide.

Base pair
Two bases in separate strands of nucleic acid that have hybridised to each other.

Carrier
Usually refers to an individual who is heterozygous for a recessive, disease-causing allele. A carrier of such an allele usually shows no symptoms of the disease but can pass the mutant allele on to his or her children.

Chromatin
The complex of DNA and proteins that makes up the chromosomes

Chromosomes
The structures within cells that carry the genetic information in the form of DNA.

Clone
A set of genetically identical organisms.

Codon
The basic 'word' in DNA. A codon contains three bases and is translated into a single amino acid.

Complementary sequences
Two sequences of DNA that will hybridise. That is, where one sequence has an adenine, the other has a thymine; where one has a guanine, the other has a cytosine.

Congenital
A condition present at birth. Congenital defects may be genetic, or caused by environmental factors, or both

Cytoplasm
One compartment of cells. The cytoplasm does the work of the cell; the nucleus stores the DNA.

Cytosine
One of the nucleotide bases in DNA and RNA.

Deletion
A mutation that involves the removal of some bases.

Deoxyribonucleic acid
The nucleic acid that is used by cells to store information.

Diploid
The number of chromosomes in most somatic cells

DNA
See Deoxyribonucleic acid.

DNA-based test
A genetic test that examines DNA directly.

Dominant
Inheritance of a mutation from one parent only (or arising anew during egg or sperm formation) can be sufficient for the person to be affected.

Double helix
DNA forms a double helix when two strands hybridise. The structure looks like two springs that have been pushed together.

Duplication
A mutation that involves the copying of a few or many bases.

Dynamic mutation
A mutation that can change on transmission from one generation to the next. The most common dynamic mutations are triplet repeat mutations.

Egg
A sex cell produced by females. The egg cell is much larger than the sperm. Only the egg cell will pass on mitochondria to the embryo.

Embryo
A developing organism that cannot live independently. In humans, the term is generally used between fertilisation and implantation; after this, the term fetus is more usual.

Enhancer
A DNA sequence that regulates expression of a gene

Enzyme
A biological catalyst, usually a protein, that carries out chemical reactions in the cell, such as producing energy, copying DNA and making more protein.

Eugenics
Attempts to "improve" a population genetically by selective breeding

Exon
Part of a gene that encodes protein (cf. Intron).

Expression
Producing protein from DNA.

Expressivity
The nature and severity of the phenotype associated with a disease genotype 

Familial
A trait that is more common within a family than in the general population; can be due to genetic and/or environmental causes

FISH
See Fluorescence in situ hybridisation.

Fluorescence in situ hybridisation
A technique that uses hybridisation and fluorescent labels to identify specific pieces of DNA in cells.

Frameshift mutation
A mutation that puts the triplet genetic code out of register (for example, insertion or deletion of one or two base pairs)

Functional genomics
Analysis of gene function on a large scale

Gamete
A sex cell (sperm or egg)

Gene
A part of the DNA molecule of a chromosome that encodes (directs the synthesis of) a protein.

Gene expression
The process by which the information in the DNA sequence of a gene is turned into protein.

Gene therapy
Treating a disease by altering the genetic make-up of the diseased cells or tissue

Genetic disease
A disease that is caused by a genetic malfunction.

Genetic heterogeneity

Refers to diseases, conditions or other characteristics that appear to be similar but whose genetic basis is different in different populations or individuals.

Genetic screening
Carrying out a genetic test on a whole unselected population, or on all the members of a subset of the population (for example, people from a particular ethnic group, pregnant women or newborn infants).

Genetic test
A test to detect the presence or absence of, or a change in, a particular gene, chromosome or gene product.

Genetics
The study of biological information and how it is inherited.

Genome
All the genetic material of an organism.

Genomics
The study of genomes.

Genotype
The specific genetic constitution of an individual.

Germ line
The sex cells and their precursors

Germ-line gene therapy
Curing a genetic disease in a heritable fashion by correcting the causative mutation in the cells of the germ line. Germline gene therapy is currently illegal

Guanine
One of the nucleotide bases in DNA and RNA

Haploid
The number of chromosomes in the sex cells (gametes). This is half the diploid number.

Haplotype
A series of alleles found at linked loci on a single chromosome

Heritability
The degree to which a characteristic is determined by genetics (as opposed to environment).

Heterozygote
Someone who is heterozygous for the gene being considered. An individual who carries two different mutant alleles in the same gene is said to be a compound heterozygote.

Heterozygous
Carrying two different alleles of a particular gene.

Homologous chromosomes
Everyone has two copies of each chromosome (with the exception of the sex chromosomes); one member of each pair is inherited from the father and the other from the mother. The members of a pair of chromosomes are known as homologous chromosomes, or homologues.

Homozygote
Someone who is homozygous for the gene being considered.

Homozygous
Carrying two identical copies of a particular gene.

Hybridisation
Describes two complementary strands of DNA binding together along their length.

In vitro fertilisation
A process that allows some childless couples to have children by bringing the eggs and sperm together outside the woman's body.

Independent segregation
During meiosis, the homologous chromsomes are split between the daughter cells independently of the members of other pairs. This means that the 46 chromosomes are shuffled in the offspring, rather than being identical to the sets of chromosomes inherited from the original person's father or mother.

Inheritance
The process by which the characteristics of organisms are passed on to their children.

Insertion
A mutation that involves the extra bases being inserted into the DNA molecule.

Intron
Part of a gene the interrupts the coding sequence (cf. exon).

Inversion
A mutation that involves a stretch of bases having their order reversed.

IVF
See In vitro fertilisation.

Junk DNA
Part of the genome that performs no known function.

Karyotype
The chromosome consitution of an individual (often displayed pictorially)

Linkage
Two genes or markers that are close together on a chromosome are rarely separated by recombination and are said to be linked. See also Linkage analysis.

Linkage analysis
Linkage analysis is a statistical method for detecting the linkage between a disease and markers of known location by following their inheritance in families. It is used to locate genes that are responsible for diseases by seeing which known markers they are close to.

Locus
The location of a gene or a marker on a chromosome.

Locus heterogeneity
The same (or similar) disease caused by mutations in different genes

Loss of heterozygosity
Loss of an allele at a locus that is heterozygous in a person's normal DNA. Loss of heterozygosity is frequently observed in tumour cells.

Marker

A gene or other segment of DNA whose position on a chromosome is known and whose inheritance can be monitored.

Meiosis
The specialised cell division that takes place when sex cells (sperm or egg cells) are produced. The members of each homologous pair of chromosomes separate from each other so that each sex cell receives only one member of each pair.

Mendelian
A term used to describe inheritance that follows the rules deduced by Mendel.

Messenger RNA
A molecule of RNA whose sequence is complementary to the coding sequence of a gene. Messenger RNA is used to make protein

Mitochondria
A component of the cell. Mitochondria produce the cell's energy and also contain a small amount of DNA (the only DNA in the cell outside the nucleus).

Mitosis
The process of cell division that occurs in somatic cells.

mRNA
See Messenger RNA.

Multifactorial
A condition determined by both genetic and environmental factors

Mutation
A change in a gene or chromosome that causes a disorder or the inherited susceptibility to a disorder.

Mutation scanning
Techniques to detect alterations between a normal sequence and a sequence containing a mutation

Mutation testing
Techniques to detect a specific mutation or mutations within a sequence

Nucleic acid
Large molecules found in the nucleus of cells, consisting of a chain of nucleotides. See DNA and RNA.

Nucleotide
The basic component from which DNA and RNA are made. In DNA, a nucleotide consists of a base (adenine [A], guanine [G], cytosine [C] or thymine [T]) linked to the sugar deoxyribose and a phosphate group. In RNA, the sugar is ribose, and the base thymine is replaced by uracil (U).

Nucleus
One compartment of cells. The cytoplasm does the work of the cell; the nucleus stores the DNA.

Pedigree
A diagram of a family history indicating the family members, their relationship with the proband, and their status with respect to a particular hereditary condition

Penetrance
The likelihood that a person carrying a mutation will develop the characteristics caused by that mutation.

Pharmacogenetics
The use of genetic information in the discovery, design and testing of pharmaceuticals.

Phenocopy
A phenotype produced by environmental factors that mimics a genetically determined trait

Phenotype
The observable traits of an organism. The phenotype results from the combination of genetic and environmental factors.

Plasmid
An independently-replicating circular piece of DNA that will survive in bacteria.

Polygenic
A characteristic (or disease) whose genetic contribution comes from more than one gene

Polymorphism
Variation in a region of DNA sequence between different individuals; the variation should be present in at least 1-2% of the population to be considered a polymorphism.

Preconceptual (or preconceptional) testing
Genetic testing of a person or couple before pregnancy

Predisposition testing
Also known as susceptibility testing. Genetic testing to establish whether a person carries a gene variant(s) that increases their risk of developing a particular disease

Preimplantation testing
Genetic testing of embryos during IVF.

Presymptomatic testing
Also known as predictive testing. The use of genetic testing to predict the future development of a disease.

Probe
A known, labelled DNA or RNA sequence that is used to detect the presence of a complementary sequence by hybridisation

Protein
A large molecule made up of amino acids. Proteins do almost all the work in cells, including forming structures and performing chemical reactions.

Proteomics
Large-scale analysis of all the proteins produced by different cell- and tissue types

Recessive
A recessive mutation has to be inherited from both parents in order for a person to be affected. Such parents are usually unaffected carriers.

Recombination
During meiosis, homologous chromosomes exchange segments, shuffling the combinations of alleles on each one. This process is called recombination and ensures that sexual reproduction passes on a mixture of alleles from all the parental chromosomes.

Recurrence risk
The probability that a genetic disorder that has occurred in a family will recur in another member in the same or a future generation

Regulatory sequence
A stretch of DNA that does not encode a protein but that affects the expression of other genes.

Replication
The term for copying DNA.

Ribonucleic acid
Nucleic acid used in cells to carry information out of the nucleus and for some other tasks.

RNA
See Ribonucleic acid.

Segregation
At meiosis, the two corresponding alleles of a gene, located on a pair of homologous chromosomes, separate (or segregate) with these chromosomes so that each sex cell (sperm or egg) receives only one of the alleles.

Sequencing
Determining the sequence of nucleotide bases along a piece of DNA.

Sex cell
Sperm and eggs. These cells are produced by meiosis and can combine to produce a new individual.

Sex chromosomes
The chromosomes that determine the sex of an individual. Human females have two X chromosomes and males have an X and a Y chromosome.

Single nucleotide polymorphism
A DNA sequence variation that involves a change in just a single nucleotide.

SNP
See Single nucleotide polymorphism.

Somatic
The normal cells of the body, excluding the germ line cells

Sperm
The male sex cells.

Splicing
The removal of introns from RNA after transcription (also used more generally to describe the end-to-end joining of two pieces of DNA by a ligase enzyme)

Stem cell
A cell that can act as a precursor to differentiated cells but which retains the capacity for self-renewal

Susceptibility testing
See predisposition testing

Template
Describes a DNA strand that is to be copied.

Therapeutic cloning
Medical and scientific applications of cloning technology that do not result in the production of genetically identical fetuses or babies

Thymine
One of the nucleotide bases in DNA (not found in RNA).

Transcription
The process by which a molecule of RNA is synthesised from a DNA template sequence, usually a gene. The RNA is then processed into messenger RNA, which is translated to produce a protein.

Translation
The process by which the sequence of a messenger RNA molecule is used to direct the order of assembly of amino acids to make a protein.

Translocation
A mutation in which a piece of DNA is moved from its normal chromosomal location to an entirely different one.

Triplet repeat
A sequence consisting of repeats of the same three nucleotides (e.g. CAG)

Trisomy
Having three copies of a specific chromosome instead of the normal two

Uracil
One of the nucleotide bases in RNA (not found in DNA).

X chromosome
One of the two sex chromosomes of humans. Females have two X chromosomes and males have one X and one Y chromosome.

X-linked inheritance
Males have only one allele of (almost) every gene on the X chromosome, so a recessive mutation in one of those genes can cause disease. Inheritance of the disease is said to be X-linked. Examples include haemophilia and X-linked colour blindness.

Y chromosome
One of the two sex chromosomes of humans. Females have two X chromosomes and males have one X and one Y chromosome. The Y chromosome carries a male-sex-determining gene that initiates development as a male.

Zygote
The diploid cell resulting from union of the egg and sperm at fertilisation