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.
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.
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.
Duplication when part of the genetic material on a chromosome is repeated.
Gene the basic unit of heredity; a segment of DNA which contains the information for a specific characteristic or function.
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 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.
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.
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.
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.
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.
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.
Protein substances which are major components of the body structure, essential to body function. They are made of smaller units called amino acids.
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.
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.