Glossary of Genetic Terminology
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D
DEAFNESS (HEARING LOSS):
May be conductive (structural problems within the ear affecting hearing) or sensorineural (problems with the tiny sensory hair cells in the cochlea, or the auditory nerve, or in the processing of sound in the brain). Deafness may be inherited (see Genetics Fact Sheet 60).
DELETION:
The loss of some genetic material from a chromosome. If the deletion is large, it may be observed in the karyotype as missing chromosomal material. If it is small, it may only be detected by analysing the composition of the DNA.
DEOXYRIBONUCLEIC ACID:
Please see 'DNA'.
DIABETES MELLITUS (DIABETES):
A condition resulting in high levels of sugar (glucose) in the blood. There are several types of diabetes mellitus and some may have a genetic component (see Genetics Fact Sheet 57).
DIAGNOSIS:
The identification and naming of an individual's disease or condition.
DIAGNOSTIC AND STATISTICAL MANUAL OF MENTAL CONDITIONS:
A handbook used by mental health professionals to diagnose mental conditions.
DIAGNOSTIC TEST:
A term used to describe particular tests that are able to identify (diagnose) a recognised condition. Prenatal diagnostic tests include; CVS, amniocentesis and some ultrasound imaging.
DIFFERENTIATE:
The process cells undergo whereby they mature and develop the features of a specialised cell with a specific function such as a heart, liver, or muscle cell.
DIGESTIVE TRACT:
Also called the gastrointestinal (GI) tract, is the multi-organ system that takes in food, extracts nutrients and expels waste.
DIPLOID NUMBER:
This is the number of chromosomes in the somatic (body) cells. There are two copies of each chromosome. In humans, the diploid number is 46.
DISCORDANT:
The situation where both members of a pair of twins do not exhibit the same characteristics. Opposite to concordant.
DISOMY:
Meaning 'two bodies'. Where there are two copies of each chromosome. This is the normal chromosome complement.
DIZYGOTIC TWINS:
Nonidentical twins, arising from two different eggs fertilised by two different sperm. Such twins are also referred to as fraternal twins.
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 twist around each other to form a double helix.
DNA FINGERPRINT:
The pattern of sequences of DNA base pairs at certain sites on a chromosome. The DNA fingerprint is as unique as possible for each individual. Specialised DNA testing to determine an individual's DNA fingerprint may be used as evidence in criminal law cases (see Genetics Fact Sheet 22).
DNA SEQUENCING:
Determining the pattern or order in which the nucleotide bases occur in a piece of DNA. This sequence is the genetic code.
DOMINANT:
Every cell contains two copies of each gene. Where only one of the gene copies or allele is mutated, and the other allele is 'correct', but the person is affected by a genetic condition due to that mutation, the mutation is described as dominant. The mutated gene is said to be dominant over the other 'correct' copy of the gene. A condition or characteristic caused by a dominant gene mutation only requires one of the genes to be mutated for the person to be affected.
DOUBLE HELIX:
The structural arrangement of DNA. The bases (basic chemical building blocks) pair up to form rungs on a ladder twisted into a helix, or coil.
DOWN SYNDROME:
Also known as trisomy 21, is a chromosomal condition that is most commonly the result of a whole extra copy of chromosome 21 found in all cells of the body. Characteristics may include intellectual delay, distinct facial features and problems with heart and digestive tract (see Genetics Fact Sheet 28).
DUCHENNE MUSCULAR DYSTROPHY (DMD):
A genetic condition that causes severe weakness, delayed walking and other problems. It usually occurs in boys (see Genetics Fact Sheet 41).
DUODENAL ATRESIA:
Refers to a blockage in the hollow tube that connects the stomach to the rest of the intestine.
DUPLICATION:
A part of the chromosome is present in two or more copies. If the duplication is large it may be observed under the microscope as a change in a chromosome; a small duplication may only be observed by examining the DNA structure of the chromosome or a gene.
DYSMORPHOLOGY:
Comes from the Greek words DYS - meaning abnormal, disease, faulty, impaired, and MORPHOLOGY - meaning structure or form. Refers to the changes in the usual structure of a person's cells, in particular, external appearance.
DYSPLASIA:
Abnormal development, or growth, of tissues or cells.