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Genetics Glossary

Allele alternative form of a gene found at the same position on pairing chromosomes. One allele is inherited from each parent.

Amino acid small chemical building blocks that join together to form proteins: there are 20 common amino acids which join together in different combinations to make up proteins.

Amniocentesis a procedure for obtaining amniotic fluid and cells for prenatal diagnosis. Using a sterile needle, a sample of amniotic fluid is removed from the uterus. The amniotic fluid contains cells from the foetus that can be analysed to determine if the foetus has a specific condition.

Autosome any of the 22 non-sex chromosomes (i.e. a chromosome that is not an X or a Y).

Autosomal dominant inheritance pattern the inheritance pattern whereby one copy of a gene is mutated; this is sufficient to cause the disease to be manifested.

Autosomal recessive inheritance pattern the inheritance pattern whereby both copies of the gene are mutated and the person develops the condition because they have no normal copy. Carriers of recessive conditions are usually unaffected.

Carrier person who is heterozygous for a recessive gene.

Cascade testing identification within a family of carriers of a particular mutation following the finding of the first affected family member.  

Chorionic villus sampling a procedure for obtaining cells of the chorionic villi to enable testing of the foetus for specific conditions.   

Chromosomes thread-like structures found in the nucleus of all body cells (except red blood cells) which is composed of DNA that carry genetic information. 

Clinical genetics the branch of the health service that is chiefly involved in diagnosis for genetic conditions and genetic counselling for individuals and families.

Congenital any abnormality, whether genetic or not, that is present from birth.

Consanguinity relationship between blood relatives.

Consultand the person presenting for genetic advice.

Deletion a mutation at the DNA level which causes a loss of genetic material.

De novo mutation a new mutation arising in a foetus that was not present in the parents.

Diploid the condition in which the cell contains two sets of chromosomes. Normal state of somatic cells in humans where the diploid number (known as 2N) is 46. 22 pairs of autosomes and a pair of sex chromosomes.

DNA (= deoxyribonucleic acid) the chemical compound that makes up genes within chromosomes and is the basic material of heredity.  It is made up of chemicals called nucleotide bases, linked together in a chain. Two chains of nucleotides twists around each other to form a double helix.

DNA testing the specific method of analysing DNA to determine if someone has a genetic disorder, will develop one or has an increased chance of developing one, or is a carrier.  

Dominant a trait that is expressed in individuals who are heterozygous for a particular allele.

Duplication when part of the genetic material on a chromosome is repeated.  

Epigenetic heritable changes to gene expression that are not due to differences in the genetic code.

Familial refers to any condition which is commoner in relatives of an affected individual than in the general population.

Gametes refers to sperm cells in males and the egg cells in females.

Gene the basic unit of heredity; a segment of DNA which contains the information for a specific characteristic or function.

Gene therapy treatment of inherited disease by addition, insertion or replacement of a normal gene or genes.

Genetics the scientific study of heredity and variation.

Genetic code the information contained in the DNA which is ‘interpreted’ by the cells to produce proteins. The chemicals (nucleotides) which make up the DNA can be described by the letters A (Adenine), T (Thymine), C (Cytosine), and G (Guanine). Thus the genetic code can be written as a series of letters (for example AAA CGT TTC).

Genetic counselling diagnosis, information and support provided by health professionals with specialised training in genetics and counselling.

Genetic predisposition due to their inherited makeup an individual may have a particular susceptibility to a condition if exposed to the correct environmental triggers.

Genetic screening the systematic search for individuals in a population who have certain genotypes.

Genetic susceptibility an inherited predisposition to a disease or disorder that is not due to a single-gene cause and is usually the result of a complex interaction of the effects of multiple different genes or gene-environment interactions.

Genetic testing the use of methods to determine if someone has a genetic disorder, will develop one or has an increased chance of developing one, or is a carrier.  Such methods range from testing at the molecular level (e.g. DNA testing) or may test protein product or cellular level.

Genome the complete set of genes carried by a cell or individual.

Genotype the genetic constitution of an individual.

Germ cells the cells of the body that transmit genetic information to the next generation (egg and sperm).

Haploid this is the number of chromosomes in the sex cells (sperm or egg). There is one copy of each chromosome. In humans, the haploid number is 23.

Heredity the transmission of characteristics to descendants.

Heterozygous (= carrier) the presence of two different alleles at one particular locus on a pair of homologous chromosomes. Carrier state for a mutated gene

Homologous chromosomes that pair during meiosis, one inherited from the mother, one from the father. Partner chromosomes.

Homozygous the presence of two identica lalleles at a particular locus on a pair of homologous chromosomes. An individual can be homozygous for the correct copies of the gene or can be homozygous for the mutated copies of the gene.

Inherited the transmission of genetic information from a parent to a child.

Insertion the addition of a piece of chromosomal material into a chromosome in a place where it is not normally found. This may result in a genetic condition, because the genetic code may be read or translated incorrectly.

Karyotype the term used to describe an individual’s chromosomes that have been photographed through a microscope and then arranged according to a standard classification based on their group and size.

Linkage the tendency for genes or segments of DNA which are located close together on the same chromosome to be inherited together.

Locus the position occupied by a gene on a chromosome.

Meiosis the process of cell division which only takes place in reproductive cells and results in eggs and sperm cells which contain 23 chromosomes (the haploid number).

Mitosis the process of cell division which takes place in all cells (somatic cells) except the reproductive cells. Mitosis results in ‘daughter’ cells which are genetically identical to the parent cells.

Multifactorial inheritance a pattern of inheritance which results from the interaction of one or more genes with environmental factor(s).

Mutation a permanent change in genetic material, either of a single gene, or in the number or structure of the chromosomes. A mutation that occurs in the gametes is inherited; a mutation in the somatic cells is not inherited. Mutations can occur naturally and spontaneously or they may be due to exposure to mutagens. The mutation may cause a change in the protein product of the gene and therefore cause health problems for the person concerned.

Nucleotides the basic components of DNA. The nucleotides are denoted by the letters A (adenine), G (guanine), C (cytosine) and T (thymine). The sequence of these nucleotides forms a genetic code

Oncogene a gene affecting cell growth or development that can cause cancer.

Pedigree a diagrammatic representation of a family health history which uses a standard set of symbols.

Pharmacogenetics the study of genetically determined variation in drug metabolism.

Phenotype the clinical manifestation (signs and symptoms) of the condition.

Polymorphism the occurrence in a population of two or more genetically determined forms in such frequencies that the rarest of them could not be maintained by mutation alone.

Predictive testing genetic testing which looks for the presence of a mutation in a gene prior to an individual developing any symptoms of the condition to determine if an individual is at increased risk of developing the condition in the future. The detection of a specific mutation does not necessarily mean the individual will develop the condition. An example is testing for the breast and ovarian cancer susceptibility genes.

Preimplantation genetic diagnosis (PGD) the genetic testing of embryos created by in vitro fertilisation (IVF) prior to implantation in the uterus.

Prenatal diagnosis the use of tests during a pregnancy (such as amniocentesis or chorionic villus sampling) to determine whether an unborn child is affected by a particular disorder.

Presymptomatic testing genetic testing which looks for the presence of a mutation in a gene prior to an individual developing any symptoms of the condition. If a mutation if found the individual is more than likely to develop the condition in the future. An example is testing for Huntington disease.   

Proband an affected individual through whom a family comes to the attention of an investigator.

Protein substances which are major components of the body structure, essential to body function. They are made of smaller units called amino acids.

Recessive a trait that is expressed in individuals who are homozygous for a particular allele.

Recurrence risk the risk that an inherited condition will occur again in a family.

Sex chromosomes an X or a Y chromosome which determine whether an individual is male or female. XX female and XY male.

Somatic cells all the cells of the body except the reproductive cells (sex cells).

Sporadic a mutation that results in a genetic condition which appears for the first time in a family. The mutation takes place in either the egg or the sperm or at conception.

Translocation the transfer of genetic material from one chromosome to another chromosome.

Tumour suppressor gene a term to describe genes that appear to prevent the development of certain types of tumours.

X-linked inheritance pattern a pattern of inheritance whereby the mutated gene is on the X chromosome, of which males have one copy and females have two. If recessive condition tends to just affect males.

Page last updated on 19 May 2014