Medical terms are usually formed of a number of parts. Knowledge of how terms are built up help in the understanding of medical terminology. For example, hyper- means excessive or increased and hypo- means deficient or decreased, so hypertension means higher than normal blood pressure and hypotension is lower than normal blood pressure.
General medical dictionaries
Children First for Health
http://www.childrenfirst.nhs.uk/kids/hospital/dictionary
Clear descriptions of a range of medical terms aimed at children.
MedlinePlus
http://www.nlm.nih.gov/medlineplus/mplusdictionary.html
Short explanation of medical terms from the US government site MedlinePlus.
Genetic glossaries and information
National Human Genome Research Institute
http://www.genome.gov/glossary.cfm
A glossary from the National Human Genome Research Institute.
Genetic Alliance UK
http://www.geneticalliance.org.uk/glossary.htm
A glossary from the Genetic Alliance UK.
Glossary of medical terms used
Abnormal: a variation from the norm. In the case of genes, an abnormal gene may result in a specific disorder.
Acquired: a condition or disease originating after birth. An acquired condition is not inherited.
Acute: a sudden onset of symptoms or disease, which may be severe and/or brief in duration.
Allele: one of the alternative versions of a gene at a given location (locus) along a chromosome.
Autosomal: any chromosome of the pairs 1-22, other than the X and Y (sex chromosomes). If a genetic condition is autosomal it can affect males or females equally.
Autosomal dominant inheritance: see the Patterns of Inheritance section.
Autosomal recessive inheritance: see the Patterns of Inheritance section.
Cell: the basic structural unit of all living organisms. It is surrounded by a membrane and contains a nucleus that carries genetic material.
Chorionic villus sampling: a method of collecting the chorion cells from a pregnant woman. Chorion cells are situated on the wall of the uterus (womb). They have the same origin as the fetal cells and can, therefore, be analysed to detect problems with a baby developing in the womb.
Chromatid: the two halves into which a chromosome is longitudinally divided during mitosis and meiosis.
Chromosome: a rod-like structure present in the nucleus of all cells in the body (with the exception of the red blood cells), which stores genetic information.
Chronic: long-term or prolonged disease or condition that can persist or progress over a long period of time.
Congenital: a condition that is present at birth, although it can be recognised prenatally, at birth or many years later. A congenital disorder can be because of a genetic defect or acquired at any time during fetal development.
Deletion: the absence of genetic material on a chromosome.
DNA: deoxyribonucleic acid is the ‘building block’ for all genetic material. Stretches of DNA compose our genes, which code for proteins that carry out essential functions in our body.
Duplication: occurs where a chromosome or part of a chromosome is duplicated. This may happen during cell division prior to fertilisation.
Enzyme: a protein that is essential to the correct functioning of biochemical reactions. Examples of these reactions include the breaking down of food into essential chemicals required by the body and the breakdown of toxic by-products in our bodies.
Familial: characteristic of some or all members of a family.
Fragile site: a site on a chromosome where genetic material may be prone to break.
Genetic counselling: information and support provided to parents who have known conditions in their family or who are concerned about the future possibility of genetically transmitted disorders.
Gene therapy: the insertion, alteration or removal of genes to treat disease. The most common form of gene therapy involves the insertion of functional genes in order to replace a mutated gene. Gene therapy is in its infancy.
Heterozygosity: the presence of different alleles at one or more loci on homologous chromosomes.
Homologous chromosomes: a pair of chromosomes containing the same linear gene sequences, each derived from one parent.
Homozygote: an individual with both identical alleles (versions of a single gene) at one locus (position).
Idiopathic: a disease or syndrome that is of unknown cause.
Imprinting: refers to the phenomenon whereby the effect of inheriting a gene is different depending on whether the inheritance was from the mother or father.
Inversion: occurs where a chromosome breaks in two and becomes reattached after turning round 180 degrees.
Intravenous: delivered through a vein.
In vitro fertilisation: in vitro fertilisation (IVF) is a process by which an ovum is fertilised by sperm outside the body.
Locus (plural loci): the position on a chromosome of a gene or other chromosome marker and also the DNA at that position.
Meiosis: cell division to produce sex cells (ovum and sperm). Sometimes called ‘reductive division’ as the number of chromosomes in each cell is halved.
Mitochondria: refers to the small bodies that are responsible for energy production in human cells. Mitochondria also carry their own genes and DNA.
Mitosis: division of all cells (except the reproductive cells), which results in the daughter cells having the same chromosome number and genetic make-up as the parent cell.
Monosomy: the total loss of one of a pair of chromosomes. This occurs, for example, in Turner syndrome where one X chromosome is lost leaving a total of 45 chromosomes.
Mosaicism: where a genetic or chromosomal abnormality does not occur in all body cells. The proportion of normal to abnormal cells will determine the severity of the disorder.
Mutant: a gene which has undergone change from the normal gene. This mutation may be an established one or a new 'sporadic' mutation.
Neural tube defect: occurs when there is a defective development of the spinal cord and the brain in the developing baby. Spina bifida is an example a developmental defect.
Nucleus: a body in the centre of each cell that contains the chromosomes with their genetic material.
Ovum: the female reproductive cell, also known as an egg.
Pattern of inheritance: describes how a gene is inherited, for example, autosomal dominant, autosomal recessive or X-linked.
Penetrance: the degree to which an individual who carries a dominant gene may show symptoms of the disorder.
Placenta: a complex structure occurring in pregnancy. It is attached to the wall of the womb and connected to the fetus by the umbilical cord. The fetus receives its nourishment through the placenta and the vessels of the umbilical cord.
Predisposition (genetic): that the individual concerned is intrinsically more prone to develop a particular disorder.
Preimplantation genetic diagnosis: when testing is performed on an embryo prior to in vitro fertilisation to determine if a genetic abnormality is present.
Ring formation: occurs where the ends of a chromosome bend over and fuse together with a loss of genetic material.
Sperm (abbreviation of spermatozoon): The male reproductive cell.
Subcutaneous: located or placed just beneath the skin.
Translocation: occurs where there is a rearrangement in which a piece of one chromosome is transferred to another with a different number.
Tripoldy: the presence of a full extra set of chromosomes. This abnormality is invariably lethal and would lead to a miscarriage or stillbirth.
Trisomy: the addition of a complete extra chromosome to a pair.
Twins: may be genetically identical (monozygous) when they arise from a single fertilised egg or non-identical (dizygous) when they arise from two separate eggs.
X-linked inheritance: the form of inheritance where the abnormal gene is carried on the X chromosome.
Information last updated April 2012.
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