T

Tandem (DNA) repeats: A series of repeated sequences, arranged next to each other.
Tandem duplication: A duplication mutation in which the duplicated DNA is found next to the original DNA.
Tautomerization: The spontaneous isomerization of a nitrogen base from its normal keto (or amino) form to an alternative hydrogen-bonding enol (or imino) form.
Taxon (plural: taxa): Taxon is a group of organisms that is recognized as a formal unit. For example, Homo sapiens is a taxon at the species level, while Homo is a taxon at the genus level.
Taxonomy: Classification of living organisms, also known as systematics.
Telocentric: A chromosome with the centromere in the middle.
Telomerase: An enzyme complex found in most eukaryotes that maintains the lengths of telomeres through successive divisions.
Telomere: The end of a chromosome.
Tetraploid: Carrying four copies of each chromosome gene.
Theory: A set of interconnected hypotheses that leads to testable predictions.
Thermophile: An organism with a growth temperature optimum between 50-80 degree centigrade.
Thermoregulation: The maintenance of a particular temperature of the living body.
Tip: The terminal node on a phylogenetic tree.
Totipotent: Refers to the ability of a cell to divide and differentiate into any cell type of both embryonic and extra-embryonic origin.
Tracheid: A cell with strengthened walls that functions to transport fluid within plants.
Trade-off: A situation where one trait cannot be increased without decreasing another. The term is used to describe constraints in optimization arguments.
Trait: characteristic.
Transcription: Replication of an RNA strand complimentary to a DNA sequence.
Transcription factor: A molecule that binds to the promoter and regulates transcription.
Transduction: The movement of genes form a donor cell to a recipient cell with a virus as the vector.
Transfer RNA (tRNA): An RNA molecule that couples a specific amino acid to a specific sequence of three bases. It is responsible for translating the genetic code.
Transformation: The introduction of a fragment of DNA into a genome. Transformation occurs naturally in some bacteria and archaea; in the laboratory, it is the basis of genetic engineering.
Transgenic: Genetically manipulated to carry genes from another individual or species.
Transition: A mutation in which a purine replaces another purine or a pyrimidine replaces another pyrimidine.
Translation: Synthesis of protein with amino acid sequence encoded by an RNA sequence.
Translocation: A rearrangement mutation in which part of one chromosome breaks away and joins another.
Transmission disequilibrium test (TDT):  A statistical test that detects association between genetic markers and disease alleles by looking for marker alleles that are transmitted in excess to affected offspring.
Transposable element: A genetic element that can move from on location in the genome to another. Also called transposon.
Transversion: A mutation in which a purine replaces a pyrimidine or vice versa.
Tree of life: A phylogenetic tree showing the relationship among all cellular organisms.
Trend: A general course or prevailing tendency. Line indicating upward or downward movement over a period of time.
Trigonotarbid: Member of an order of extinct terrestrial spider-like animals (order Trigonotarbida).
Trimerophyte: Member of an early group of vascular plants.
Triploid: Carrying three genomes.
Trisomy: Possession of three copies of one chromosome (in humans is abnormal because there are normally only two copies).
Tristyly: A polymorphism with three different arrangements of anther and stigma. It promotes outcrossing.
Trocophore: Larval type characteristic of many protostomes including annelids and many mollusks.
Trophic: Pertaining to nutrition, concerned to obtaining food.
True breeding: A population or individual that produces genetically identical offspring.
Truncation selection: Selection that eliminates those with the largest (or smallest) trait values.