Genetic and Medical Glossary

Glossary

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There are 40 names in this directory beginning with the letter T.
T
the abbreviation for thymine, a pyrimidine nitrogenous base found in the structure of DNA; in RNA, thymine is replaced by uracil

Tandem repeats
sequence of two or more DNA base pairs that is repeated in such a way that the repeats lie adjacent to each other on the chromosome

Telecanthus
abnormally increased distance between the medial canthi (corners of the eye) of the eyelids

Telocentric chromosome
a chromosome with a centromere at the end; also called an acrocentric chromosome

Telomere
the ends of a eukaryotic chromosome

Teratoma
type of germ cell tumor that may contain several different types of tissue and sometimes mature elements such as hair, muscle, and bone

Terminal deletion
deletion that occurs towards the end of a chromosome

Tetralogy of Fallot
the combination of four different heart defects: ventricular septal defect (VSD), pulmonary stenosis (obstructed outflow of blood from the right ventricle to the lungs), dextroposition or overriding aorta (displaced aorta, causing blood to flow into the aorta from both right and left ventricles); and right ventricular hypertrophy (abnormal enlargement of the right ventricle)

Tetrasomy
four copies of a particular chromosome present in a cell, resulting in 48 chromosomes in the cell instead of the usual 46

Thenar region
fleshy mass of the outer side of the palm of the hand

Thrombocytopenia
low blood platelet count

Thumb, absent
absence of both phalanges of a thumb and the associated soft tissues

Thumb, adducted
in the resting position, the tip of the thumb is on, or near, the palm, close to the base of the fourth or fifth finger

Thumb, triphalangeal
thumb with three phalanges in a single, proximodistal axis

Thymine (T)
pyrimidine nitrogenous base found in the structure of DNA; in RNA, thymine is replaced by uracil

Tongue, bifid
tongue with a median apical indentation or fork; may be associated with ankyloglossia

Tongue, large
increased length and width of the tongue; leads to protrusion of the tongue

Tooth, natal
erupted tooth or teeth at birth

Tooth, supernumerary
extra tooth or teeth

Townes–Brocks syndrome
autosomal dominant genetic disorder characterized by absence of the anal opening (imperforate anus), abnormal ears associated with hearing impairment and thumb malformations; abnormalities in the feet, heart and kidneys also occur frequently. Townes-Brocks syndrome is associated with a mutation in the SALL1 gene located on the long arm of chromosome 16 at 16q12.1

Tracheaesophageal fistula
birth defect in which the trachea is connected to the esophagus

Trait
phenotypic characteristic that can be inherited

Transcription
cellular process by which genetic information in the form of a gene in a DNA molecule is converted into the form of a messenger RNA (mRNA) molecule

Transfer RNA (tRNA)
type of RNA molecule necessary for translation to occur properly; provides the basis of the genetic code, in which codons in a messenger RNA (mRNA) molecule are used to direct the sequence of amino acids in a polypeptide; contains a binding site for a particular amino acid and a region complementary to a messenger RNA (mRNA) codon (an anticodon)

Transition mutation
DNA mutation in which one pyrimidine (cytosine or thymine) takes the place of another, or a purine (adenine or guanine) takes the place of another

Translation
cellular process by which genetic information in the form of a messenger RNA (mRNA) molecule is converted into the amino acid sequence of a protein, using ribosomes and RNA molecules as accessory molecules

Translocation
movement of a chromosome segment to a nonhomologous chromosome as a result of an error in recombination

Transposition of great arteries
birth defect in which the two major vessels that carry blood away from the heart the aorta and the pulmonary artery are switched (transposed)

Transverse limb reduction
partial or complete absence of a part of one or more limbs in the absence of other abnormalities

Tricho–rhino–phalangeal syndrome
genetic disorder consisting of fine and sparse scalp hair, thin nails, pear-shaped broad nose, and cone-shaped epiphyses of the middle phalanges of some fingers and toes

Trigonocephaly
Wedge-shaped or triangular head, with the apex of the triangle at the midline of the forehead and the base of the triangle at the occiput, assessed from above

Triploid
possessing three complete sets of chromosomes

Trisomy
genetic condition involving one chromosome of a homologous chromosome pair that has been duplicated in some way, giving rise to a 2N + 1 genotype and causing serious phenotypic abnormalities; well-known trisomies are Trisomy 13, 18, and 21

Trisomy 13
see Patau syndrome

Trisomy 18
see Edwards syndrome

Trisomy 21
see Down syndrome

Truncus arteriosus
congenital heart defect in which there is a single main blood vessel carrying blood away from the heart which then branches into other blood vessels; with no pulmonary artery to carry blood to the lungs, and aorta blood from both ventricles of the heart is mixed, resulting in some oxygen-rich blood travelling back to the lungs and some oxygen-poor blood travelling to the rest of the body

Tuberous sclerosis (TS)
genetic multisystem disorder characterized by episodes of uncontrolled electrical activity in the brain (seizures), intellectual impairment, distinctive skin abnormalities (lesions), and benign (noncancerous), tumor-like nodules (hamartomas) that appear in the brain, certain regions of the eyes (e.g., retinas), the heart, the kidneys, the lungs, or other tissues or organs; caused by mutations of either the TSC1 gene located on the long arm (q) of chromosome 9 at 9q34 or the TSC2 gene located on the short arm (p) of chromosome 16 at 16p13

Turner syndrome
a human genetic defect in which a female has only forty-five chromosomes and is lacking a second X sex chromosome; abnormalities such as sterility and anatomical defects are common

Turricephaly
tall head relative to width and length; head length and width are reduced compared to normal age-related standards

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