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A

ABNORMAL:
Any change from the 'correct' or 'usual'. It may not necessarily mean harmful or undesirable; it can equally mean atypical, unusual or uncommon. When used in reference to genes, an abnormal gene may result in a genetic condition.

ACHONDROPLASIA:
Achondroplasia is a genetic condition that results in abnormally short stature (skeletal dysplasia). Achondroplasia is the most common cause of short stature with disproportionately short limbs (see Genetics Fact Sheet 38).

ACROCENTRIC:
Acrocentric chromosomes are those with the centromere very close to one end, giving the chromosome a 'V' shaped appearance. The acrocentric chromosomes are numbers 13, 14, 15, 21 and 22. The short 'p' arms are very short and usually have small round appendages on stalks, known as 'satellites'.

ADENINE (A):
One of the four basic building blocks (nucleotide bases) that makes up the genetic code or DNA. It is represented by the letter A.

ADULT STEM CELL:
An undifferentiated cell found in a differentiated tissue that can renew itself and (with certain limitations) differentiate to yield all the specialised cell types of the tissue from which it originated.

ALLELE:
There are usually two copies of a gene. These two copies are called alleles. In some cases, one or both alleles will be mutated or altered in some way.

ALZHEIMER DISEASE (AD):
Is the most common form of dementia. Individuals with dementia have a gradual build up of changes in the brain. The condition is characterised by progressive difficulties with memory, language, learning, thinking and reasoning, and undertaking everyday tasks. Changes in personality may also occur. There may be a genetic component to the cause of the condition (see Genetics Fact Sheet 45).

AMINO ACIDS:
Small chemical building blocks that join together to form proteins: there are 20 common amino acids which join in different combinations to make up proteins.

AMNIOCENTESIS:
A procedure for obtaining amniotic fluid for prenatal diagnosis. Using a sterile needle, a sample of amniotic fluid is removed from the uterus; the amniotic fluid contains cells from the fetus which can be analysed to determine if the fetus has a specific condition. The test may be carried out from the 15th week of pregnancy (see Genetics Fact Sheet 17C).

AMNIOTIC FLUID:
Fluid in which the fetus floats in the uterus; fetal cells are found suspended in this fluid.

ANEUPLOIDY:
The gain or loss of chromosomal material ie. having missing or extra chromosomes (see Genetics Fact Sheets 6 and Fact Sheet 7).

ANTIBODY:
A protein, produced in response to a foreign substance in the immune system.

ANTICIPATION:
The situation where a genetic condition appears to become more severe and/or arise at an earlier age as it is passed through subsequent generations (seen in many trinucleotide repeat mutations).

ASSISTED REPRODUCTIVE TECHNOLOGY(ART):
A term describing a variety of measures employed to increase the possibility of pregnancy. Includes IVF (In Vitro Fertilisation).

ATP:
Stands for adenosine triphosphate. ATP is a chemical that is used in cells to drive chemical reactions in which energy is produced.

AUTISM:
The autism spectrum disorders (ASDs) represent a group of developmental conditions characterised by impairments in communication and social interactions, restricted, repetitive and stereotyped behaviour and anxiety and compulsions. The vast majority (about 90% of cases) are of unknown cause. There may be a genetic component in some cases (see Genetics Fact Sheet 43).

AUTOSOMAL DOMINANT MUTATION:
A dominant mutation in a gene which is carried on an autosome.

AUTOSOMAL GENE:
Any gene which is located on an autosome.

AUTOSOMAL RECESSIVE MUTATION:
A recessive mutation in a gene which is carried on an autosome.

AUTOSOME:
Any chromosome that is not a sex chromosome (that is not an X or Y chromosome). In humans, the autosomes are the numbered chromosomes and are given the numbers 1-22. Chromosome 1 is the largest and 22 is the smallest.