Glossary of Genetic Terminology
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An inherited degenerative condition of the nervous system where apparently healthy babies at about 6 months of age lose already acquired skills, gradually become blind, paralysed and unaware of his/her surroundings and die before approximately 5 years of age. It is more common amongst people of Ashkenazi Jewish background (see Genetics Fact Sheet 35).
TELOMERE:
The terminal or end segment of each chromosome arm.
TERATOGEN:
A substance that produces or increases the incidence of birth defects or
congenital abnormalities by interfering with development of the fetus during pregnancy.
TERMINATION OF PREGNANCY:
Intervention to ensure a pregnancy does not continue.
TETRAPLOIDY:
Four copies of every chromosome resulting in 92 chromosomes in the cell, instead of the usual 46.
TETRASOMY:
Four copies of a particular chromosome present in a cell, resulting in 48 chromosomes in the cell instead of the usual 46.
An inherited blood condition in which the chains of the heamoglobin molecule (a type of protein in red blood cells that carries oxygen to the tissues) are decreased; alpha thalassaemia is where the gene for the alpha chain is not working properly, while beta thalassaemia is where the beta chain gene is faulty; signs and symptoms of the thalassaemias vary from mild (little to no symptoms) to severe (life threatening) (see Genetics Fact Sheet 34).
THROMBOEMBOLISM:
A clot in a blood vessel obstructing the flow of blood.
THROMBOPHILIA:
Please see 'Clotting condition'.
THROMBUS:
A blood clot.
One of the four basic building blocks (nucleotide bases) that makes up the genetic code (DNA). It is represented by the letter T.
Collection of interconnected cells that have a similar function in the body.
Making an RNA copy from a sequence of DNA (a gene). Transcription is the first step in producing a gene product or protein.
A gene artificially introduced into the cell or into the genome of an individual.
The process of producing a protein from mRNA by stringing together amino acids (protein chemical building blocks) in a specific order, and this is determined by the mRNA code. Translation is the second step in creating a protein from DNA, the first step being transcription.
TRANSLOCATION:
This occurs when a piece of one chromosome breaks
off and attaches to another, different chromosome. When no material is lost or
gained the translocation is said to be 'balanced' and the individual may or may not be affected by it. An 'unbalanced' translocation results in the loss or gain of genetic
material which may result in a genetic condition.
TRINUCLEOTIDE REPEAT:
Please see 'Triplet Repeat'.
TRIPLET:
A sequence of three nucleotides in the DNA sequence. Each triplet represents the code for a particular amino acid.
TRIPLET
REPEAT (TRINUCLEOTIDE REPEAT):
A form of genetic mutation which consists of a
series of repeated identical sequences of DNA triplets which may be found either inside or outside a gene. An
increase in the number of such repeats in a particular gene can lead to instability of the gene and manifestation of the corresponding
genetic condition. This form of genetic mutation has been associated with a number of genetic conditions including Huntington disease, Fragile X syndrome, Myotonic
dystrophy etc.
TRIPLOIDY:
Having three copies of every chromosome resulting
in 69 chromosomes in a cell instead of the usual 46.
TRISOMY:
Three copies of a particular chromosome are
present in a cell resulting in 47 chromosomes
instead of the usual 46.
Please see 'Patau syndrome'.
Please see 'Edward syndrome'.
Please see 'Down syndrome'.
TUMOUR SUPPRESSOR GENE:
A gene which contains the information for proteins whose role in cells is to suppress
the formation of tumours by controlling cell division and growth. Loss of this control leads
to development of malignancy. P53 is an example of a tumour suppressor gene.
Also called X0 syndrome, Turner syndrome is a chromosomal condition where females are missing one (or part of one) of their two X chromosome copies; they have a total of 45 chromosomes including one X xhromosome (45,X) instead of the ususal two copies (46,XX). This condition is characterised by short stature and the lack of sexual development in girls at puberty. Other physical features may include a short neck with a webbed appearance, heart defects and kideny abnormalities (see Genetics Fact Sheet 32).